# Diving, Snorkeling & Swim

Across masks, snorkels, fins, dive computers, and goggles, what matters shifts with the activity: scuba and snorkeling need a sealed, standards-certified mask and a dry snorkel valve, fins matched to leg strength power them, a dive computer's algorithm sets safe limits, while pool swimming only needs an anti-fog, well-fitted goggle lens.

## How it works

A mask traps a sealed air pocket behind a silicone-skirted, tempered-glass lens so a diver can see underwater and must equalize that pocket as pressure changes; a lower-volume mask needs less breath to clear. A snorkel is a surface breathing tube; a top valve keeps waves out and a purge valve lets a diver blow trapped water out without lifting their head. Fins convert a leg kick into thrust, with blade stiffness matched to leg strength and an open-heel pocket fitting over a boot where a full-foot pocket does not. A dive computer times a dive against a decompression algorithm to set safe ascent limits and can read tank pressure wirelessly. Goggles seal with a soft gasket and add tinted, mirrored, or anti-fog coatings to the lens.

## How an expert narrows the choice

```mermaid
timeline
  section 1 · Activity
    scuba_diving : skip Goggle lens tint, Goggle fit profile, Anti-fog coating
    snorkeling : skip Decompression algorithm, Air integration, Dive computer display, Goggle lens tint, Goggle fit profile, Anti-fog coating, Regulator connection type, Regulator first-stage balance, Regulator cold-water rating, Octopus (secondary) included, BCD style, BCD weight integration, BCD lift capacity class
    pool_swimming : skip Snorkel top valve, Snorkel purge valve, Snorkel mounting, Snorkel safety certification, Decompression algorithm, Air integration, Dive computer display, Mask safety certification, Mask internal volume, Regulator connection type, Regulator first-stage balance, Regulator cold-water rating, Octopus (secondary) included, BCD style, BCD weight integration, BCD lift capacity class
  section 2 · Gear type
    mask : ask Mask lens layout : skip Snorkel top valve, Snorkel purge valve, Snorkel mounting, Snorkel safety certification, Fin foot pocket, Fin blade type, Fin blade material, Fin blade stiffness, Dive computer display, Decompression algorithm, Air integration, Goggle lens tint, Goggle fit profile, Anti-fog coating, Regulator connection type, Regulator first-stage balance, Regulator cold-water rating, Octopus (secondary) included, BCD style, BCD weight integration, BCD lift capacity class
    snorkel : ask Snorkel top valve : skip Mask lens layout, Mask skirt material, Mask internal volume, Mask safety certification, Fin foot pocket, Fin blade type, Fin blade material, Fin blade stiffness, Dive computer display, Decompression algorithm, Air integration, Goggle lens tint, Goggle fit profile, Anti-fog coating, Regulator connection type, Regulator first-stage balance, Regulator cold-water rating, Octopus (secondary) included, BCD style, BCD weight integration, BCD lift capacity class
    fins : ask Fin foot pocket : skip Mask lens layout, Mask skirt material, Mask internal volume, Mask safety certification, Snorkel top valve, Snorkel purge valve, Snorkel mounting, Snorkel safety certification, Dive computer display, Decompression algorithm, Air integration, Goggle lens tint, Goggle fit profile, Anti-fog coating, Regulator connection type, Regulator first-stage balance, Regulator cold-water rating, Octopus (secondary) included, BCD style, BCD weight integration, BCD lift capacity class
    dive_computer : ask Dive computer display : skip Mask lens layout, Mask skirt material, Mask internal volume, Mask safety certification, Snorkel top valve, Snorkel purge valve, Snorkel mounting, Snorkel safety certification, Fin foot pocket, Fin blade type, Fin blade material, Fin blade stiffness, Goggle lens tint, Goggle fit profile, Anti-fog coating, Regulator connection type, Regulator first-stage balance, Regulator cold-water rating, Octopus (secondary) included, BCD style, BCD weight integration, BCD lift capacity class
    swim_goggles : ask Goggle lens tint : skip Mask lens layout, Mask skirt material, Mask internal volume, Mask safety certification, Snorkel top valve, Snorkel purge valve, Snorkel mounting, Snorkel safety certification, Fin foot pocket, Fin blade type, Fin blade material, Fin blade stiffness, Dive computer display, Decompression algorithm, Air integration, Regulator connection type, Regulator first-stage balance, Regulator cold-water rating, Octopus (secondary) included, BCD style, BCD weight integration, BCD lift capacity class
    regulator : ask Regulator connection type : skip Mask lens layout, Mask skirt material, Mask internal volume, Mask safety certification, Snorkel top valve, Snorkel purge valve, Snorkel mounting, Snorkel safety certification, Fin foot pocket, Fin blade type, Fin blade material, Fin blade stiffness, Dive computer display, Decompression algorithm, Air integration, Goggle lens tint, Goggle fit profile, Anti-fog coating, BCD style, BCD weight integration, BCD lift capacity class
    bcd : ask BCD style : skip Mask lens layout, Mask skirt material, Mask internal volume, Mask safety certification, Snorkel top valve, Snorkel purge valve, Snorkel mounting, Snorkel safety certification, Fin foot pocket, Fin blade type, Fin blade material, Fin blade stiffness, Dive computer display, Decompression algorithm, Air integration, Goggle lens tint, Goggle fit profile, Anti-fog coating, Regulator connection type, Regulator first-stage balance, Regulator cold-water rating, Octopus (secondary) included
```

1. **Activity.** Scuba diving, snorkeling at the surface, or pool swimming?
   Depth, air supply, and the standards gear must meet differ completely between the three; a scuba mask and computer are irrelevant to a lap swimmer, and a pool goggle's coatings are irrelevant to a diver.
   If scuba_diving: skip Goggle lens tint, Goggle fit profile, Anti-fog coating.
   If snorkeling: skip Decompression algorithm, Air integration, Dive computer display, Goggle lens tint, Goggle fit profile, Anti-fog coating, Regulator connection type, Regulator first-stage balance, Regulator cold-water rating, Octopus (secondary) included, BCD style, BCD weight integration, BCD lift capacity class.
   If pool_swimming: skip Snorkel top valve, Snorkel purge valve, Snorkel mounting, Snorkel safety certification, Decompression algorithm, Air integration, Dive computer display, Mask safety certification, Mask internal volume, Regulator connection type, Regulator first-stage balance, Regulator cold-water rating, Octopus (secondary) included, BCD style, BCD weight integration, BCD lift capacity class.
   Skip if already known: a query naming scuba, snorkeling, or pool/lap swimming, an earlier answer naming a dive certification level or a swim team.

2. **Gear type.** A mask, snorkel, fins, dive computer, regulator, BCD, or goggles?
   Each type has its own defining spec — a mask's seal and lens, a snorkel's valve, a fin's blade, a computer's algorithm, a goggle's lens coating — so asking this first avoids asking about a blade on a mask.
   If mask: ask Mask lens layout; skip Snorkel top valve, Snorkel purge valve, Snorkel mounting, Snorkel safety certification, Fin foot pocket, Fin blade type, Fin blade material, Fin blade stiffness, Dive computer display, Decompression algorithm, Air integration, Goggle lens tint, Goggle fit profile, Anti-fog coating, Regulator connection type, Regulator first-stage balance, Regulator cold-water rating, Octopus (secondary) included, BCD style, BCD weight integration, BCD lift capacity class.
   If snorkel: ask Snorkel top valve; skip Mask lens layout, Mask skirt material, Mask internal volume, Mask safety certification, Fin foot pocket, Fin blade type, Fin blade material, Fin blade stiffness, Dive computer display, Decompression algorithm, Air integration, Goggle lens tint, Goggle fit profile, Anti-fog coating, Regulator connection type, Regulator first-stage balance, Regulator cold-water rating, Octopus (secondary) included, BCD style, BCD weight integration, BCD lift capacity class.
   If fins: ask Fin foot pocket; skip Mask lens layout, Mask skirt material, Mask internal volume, Mask safety certification, Snorkel top valve, Snorkel purge valve, Snorkel mounting, Snorkel safety certification, Dive computer display, Decompression algorithm, Air integration, Goggle lens tint, Goggle fit profile, Anti-fog coating, Regulator connection type, Regulator first-stage balance, Regulator cold-water rating, Octopus (secondary) included, BCD style, BCD weight integration, BCD lift capacity class.
   If dive_computer: ask Dive computer display; skip Mask lens layout, Mask skirt material, Mask internal volume, Mask safety certification, Snorkel top valve, Snorkel purge valve, Snorkel mounting, Snorkel safety certification, Fin foot pocket, Fin blade type, Fin blade material, Fin blade stiffness, Goggle lens tint, Goggle fit profile, Anti-fog coating, Regulator connection type, Regulator first-stage balance, Regulator cold-water rating, Octopus (secondary) included, BCD style, BCD weight integration, BCD lift capacity class.
   If swim_goggles: ask Goggle lens tint; skip Mask lens layout, Mask skirt material, Mask internal volume, Mask safety certification, Snorkel top valve, Snorkel purge valve, Snorkel mounting, Snorkel safety certification, Fin foot pocket, Fin blade type, Fin blade material, Fin blade stiffness, Dive computer display, Decompression algorithm, Air integration, Regulator connection type, Regulator first-stage balance, Regulator cold-water rating, Octopus (secondary) included, BCD style, BCD weight integration, BCD lift capacity class.
   If regulator: ask Regulator connection type; skip Mask lens layout, Mask skirt material, Mask internal volume, Mask safety certification, Snorkel top valve, Snorkel purge valve, Snorkel mounting, Snorkel safety certification, Fin foot pocket, Fin blade type, Fin blade material, Fin blade stiffness, Dive computer display, Decompression algorithm, Air integration, Goggle lens tint, Goggle fit profile, Anti-fog coating, BCD style, BCD weight integration, BCD lift capacity class.
   If bcd: ask BCD style; skip Mask lens layout, Mask skirt material, Mask internal volume, Mask safety certification, Snorkel top valve, Snorkel purge valve, Snorkel mounting, Snorkel safety certification, Fin foot pocket, Fin blade type, Fin blade material, Fin blade stiffness, Dive computer display, Decompression algorithm, Air integration, Goggle lens tint, Goggle fit profile, Anti-fog coating, Regulator connection type, Regulator first-stage balance, Regulator cold-water rating, Octopus (secondary) included.
   Skip if already known: a query that already names one of mask, snorkel, fins, dive computer, regulator, BCD, or goggles.

## What to surface

### Activity

Whether the gear is for breathing underwater on scuba, breathing at the surface while snorkeling, or swimming laps in a pool with no breathing tube at all. Depth, air supply, and the standards gear must meet differ completely between the three; a scuba mask and computer are irrelevant to a lap swimmer, and a pool goggle's coatings are irrelevant to a diver. [1] [4]

Values: scuba_diving, snorkeling, pool_swimming.
Question to ask: Scuba diving, snorkeling at the surface, or pool swimming?

Used as: top-level split, filter. Data: derived from listing fields, backfill cost low. Look for it as: Derive from the product category breadcrumb or title keywords ('scuba', 'snorkel set', 'swim goggles').
Category/title keyword match covers nearly every listing; dive computers default to scuba_diving and goggles default to pool_swimming.

### Gear type

Which piece of equipment this is: a mask, a snorkel, fins, a dive computer, a regulator, a BCD, or swim goggles. Each type has its own defining spec — a mask's seal and lens, a snorkel's valve, a fin's blade, a computer's algorithm, a goggle's lens coating — so asking this first avoids asking about a blade on a mask. [1] [2] [3] [4] [8]

Values: mask, snorkel, fins, dive_computer, swim_goggles, regulator, bcd.
Question to ask: A mask, snorkel, fins, dive computer, regulator, BCD, or goggles?

Used as: top-level split, filter. Data: derived from listing fields, backfill cost low. Look for it as: Derive from the product title and category node directly; these seven product types rarely overlap in one listing.
Title keyword match ('mask', 'snorkel', 'fins', 'dive computer', 'regulator', 'BCD', 'goggles') covers almost every listing.

### Fin foot pocket

Whether the fin fits like a shoe directly over a bare foot (full-foot) or has an open heel held on by an adjustable strap, usually worn over a boot (open-heel). Full-foot fins are lighter and simpler for warm-water snorkeling and swimming but only fit barefoot in a fixed size; open-heel fins adjust to fit over a wetsuit boot, which is how nearly all scuba divers dive. [3]

Values: full_foot, open_heel_adjustable.
Question to ask: Fins to wear over bare feet, or over boots?

Used as: top-level split, filter. Data: on the listing, backfill cost low. Look for it as: Usually in the title ('Open Heel Scuba Fins', 'Full Foot Snorkel Fins').
Title keyword match; default to full_foot for snorkel-labeled fins and open_heel_adjustable for scuba-labeled fins.

### Fin blade type

The blade's shape: a solid paddle blade, a blade with a lengthwise split down the middle, or a single wide monofin blade worn on both feet together. Split fins feel lighter to kick and are marketed as more efficient, but controlled testing found no thrust gain over a solid paddle once the split was taped shut; a monofin is a dedicated freediving and finswimming tool, not a general one. [3]

Values: paddle_blade, split_blade, monofin.
Question to ask: A solid paddle blade, a split blade, or a monofin?

Used as: filter, shown on the card. Data: on the listing, backfill cost low. Look for it as: Usually in the title ('Split Fin', 'Monofin').
Title keyword match; default to paddle_blade when unstated, since it's the traditional standard.

### Dive computer display

How the computer shows dive data: a wrist-worn digital display, a console gauge-style unit that hangs off the regulator hose, or a dive watch that doubles as everyday wear. A wrist computer is the most common and glanceable; a console mounts with the pressure gauge and never needs its own strap; a watch-style computer is the only one a diver will also wear top-side. [4]

Values: wrist_digital, console_gauge_style, dive_watch_hybrid.
Question to ask: Worn on the wrist, mounted on a console, or a dive watch?

Used as: filter, shown on the card. Data: on the listing, backfill cost low. Look for it as: Usually clear from the product image and title ('Wrist Dive Computer', 'Console Computer').
Title/image match; default to wrist_digital when unstated, since it's the dominant modern form.

### Air integration

Whether the computer reads and displays the diver's tank pressure directly, either via a wireless transmitter on the regulator's first stage or a hose running to the computer, or not at all. An air-integrated computer puts remaining gas, no-stop time, and depth on one display so a diver stops checking a separate gauge; a wireless transmitter avoids the hose but adds a second battery to track and a pairing step before every dive. [4] [7]

Values: wireless_transmitter, hose_mounted, no_air_integration.
Question to ask: Should it read your tank pressure too?

Used as: filter, shown on the card. Data: on the listing, backfill cost low. Look for it as: Usually stated directly ('Air Integrated', 'AI Wireless').
Keyword match; default to no_air_integration for base console or entry wrist computers when unstated.

### Goggle lens tint

The lens treatment: clear, a solid color tint, a mirrored coating, or a polarized lens. Clear lenses work best indoors and in low light, a tint or mirror cuts glare outdoors, and a polarized lens cuts surface glare specifically, so the right choice depends on indoor pool versus open water or outdoor lap swimming. [8]

Values: clear, tinted, mirrored, polarized.
Question to ask: Clear, tinted, mirrored, or polarized lenses?

Used as: filter, shown on the card. Data: on the listing, backfill cost low. Look for it as: Usually in the title and shop-by-lens category tags.
Title keyword match; default to clear when unstated, since it's the base option in every goggle line.

### Goggle fit profile

What the goggle is built for: everyday recreational swimming, structured training, competitive racing (a lower-profile, smaller-socket fit), or a junior/kids size. A racing fit sits closer to the eye socket for less drag but tolerates less misalignment and fits a narrower range of face shapes than a recreational goggle, which trades a little drag for an easier, more forgiving seal. [8]

Values: recreational, training, racing_low_profile, junior.
Question to ask: Everyday swimming, training, racing, or a kids' fit?

Used as: filter, shown on the card. Data: on the listing, backfill cost low. Look for it as: Usually in the title or the retailer's shop-by-type category ('Racing Goggles', 'Junior Goggles').
Title/category keyword match; default to recreational when unstated.

### Octopus (secondary) included

Whether the set includes a secondary demand valve (octopus) for an out-of-air buddy to breathe from, or is sold as a first-stage/second-stage pair only. A buddy-breathing octopus is required gear for most recreational dive operations, so a regulator sold without one needs a second demand valve bought and fitted separately before it's dive-ready. [9]

Values: included, not_included.
Question to ask: Does it need to include an octopus?

Used as: filter, shown on the card. Data: on the listing, backfill cost low. Look for it as: Usually stated in the kit/bundle title ('Regulator Set with Octopus').
Keyword match; default to not_included for a single first-stage/second-stage listing without 'set' or 'octopus' in the title.

### BCD style

The buoyancy compensator's construction: a jacket-style BC that wraps around the torso like a vest, or a wing-style BC with a separate backplate and harness and an air cell on the back only. A jacket BC is simpler to don and holds a diver upright at the surface, while a wing and backplate puts the air cell on the back only, which helps hold horizontal trim underwater and swaps onto a bigger wing for more lift on technical dives. [10]

Values: jacket, wing_backplate.
Question to ask: A jacket-style BCD, or a wing and backplate?

Used as: filter, shown on the card. Data: on the listing, backfill cost low. Look for it as: Usually in the title or category tag ('Jacket BCD', 'Wing/Backplate BCD').
Title keyword match; default to jacket when unstated, since it's the more common recreational style.

### Mask lens layout

How many viewing windows the mask has: one wraparound pane, two separate lenses (one per eye), or a multi-window panoramic design. A dual-lens mask sits closer to the face and holds less air, so it's easier to equalize and often takes prescription inserts; a single or panoramic lens trades that for a wider, less broken-up field of view. [1]

Values: single_lens, dual_lens, multi_window_panoramic.
Question to ask: One wide lens, two lenses, or a panoramic mask?

Used as: filter, shown on the card. Data: on the listing, backfill cost low. Look for it as: Usually in the title or image ('Dual Lens Dive Mask', 'Panoramic Mask').
Title keyword match; default to single_lens when unstated, since it's the most common classic design.
Chart: position on a field-of-view-vs-internal-volume axis.

### Snorkel top valve

The mechanism at the top of the tube that limits water entering when a wave or splash passes over it: a float-operated dry valve, a splash-deflecting semi-dry guard, or a plain open (wet) top. A dry valve keeps the tube nearly flood-free on the surface but adds bulk and a float that can stick; a wet top is simplest and lightest but floods on every wave, trading convenience for bulk. [2]

Values: dry_valve, semi_dry_splash_guard, open_wet.
Question to ask: A dry-top valve, a splash guard, or a plain open tube?

Used as: filter, shown on the card. Data: on the listing, backfill cost low. Look for it as: Usually in the title ('Dry Snorkel', 'Semi-Dry Snorkel').
Title keyword match; default to open_wet when unstated, since it's the simplest and cheapest design.
Chart: position on a flood-resistance-vs-bulk axis.

### Snorkel purge valve

Whether the snorkel has a one-way valve at the bottom of the tube that lets the diver blow trapped water out without surfacing or removing the mouthpiece. A purge valve clears a flooded tube in one sharp exhale instead of the full lung-clearing blast an open tube needs, which matters most on a dry or semi-dry top that still lets some water in. [2]

Values: has_purge_valve, no_purge_valve.
Question to ask: Does it need a purge valve to clear water?

Used as: filter, shown on the card. Data: on the listing, backfill cost low. Look for it as: Usually stated in bullet points ('purge valve included').
Title/description keyword match; default to has_purge_valve for dry and semi-dry snorkels, since nearly all include one.

### Fin blade material

What the blade is made from: rubber, injection-molded thermoplastic, a fiberglass or carbon-fiber composite, or a mix of these. Composite and carbon-fiber blades flex and spring back with less wasted energy than rubber or plastic, which rewards a strong kick, but they're more fragile and cost several times more than a plastic fin. [3]

Values: rubber, thermoplastic, composite_or_carbon_fiber, mixed.

Used as: filter, shown on the card. Data: on the listing, backfill cost low. Look for it as: Usually stated in the title or spec bullets ('Carbon Fiber Fins').
Title keyword match; default to thermoplastic when unstated, since injection-molded plastic is the most common budget blade.
Chart: position on an energy-efficiency-vs-fragility axis.

### Anti-fog coating

Whether the lens carries a factory anti-fog coating that stops condensation forming on the inside of the lens. An anti-fog coating is a thin layer that wears off with rubbing or a fingertip wipe, so a 'fogs up after a few months' complaint is normal wear, not a defect, and rules out most do-it-yourself fixes beyond a proper anti-fog rinse. [8]

Values: factory_coated, uncoated_or_unstated.
Question to ask: Does it need an anti-fog coating?

Used as: filter, shown on the card. Data: on the listing, backfill cost low. Look for it as: Usually stated directly ('Anti-Fog Lens') or as a dedicated anti-fog shop category.
Keyword match; default to factory_coated for mid-range and above goggles, since it's now near-universal.

### Regulator connection type

How the first stage attaches to the tank valve: a DIN screw-in fitting or a CGA 850 yoke (A-clamp/international) clamp. DIN screws directly into the valve and rates to a higher working pressure, while a yoke clamp fits over the valve outlet and is the North American standard; the two aren't interchangeable without an adapter, so it has to match the diver's tank valve. [9]

Values: din, yoke_international.
Question to ask: DIN fitting or yoke (international) connection?

Used as: filter, shown on the card. Data: on the listing, backfill cost low. Look for it as: Usually stated directly in the title or spec bullets ('DIN Regulator', 'Yoke/INT Regulator').
Keyword match; default to yoke_international when unstated, since it's the more common recreational fitting outside DIN-standard regions.

### Regulator cold-water rating

Whether the regulator is rated or kitted for cold-water diving, where a standard regulator risks internal ice formation and free-flow. A cold-water-rated regulator resists freezing and free-flow in water near 0°C, which an unrated regulator isn't tested for and can fail at exactly when a diver needs it most. [9]

Values: cold_water_rated, not_rated_or_unstated.
Question to ask: Will this be used in cold water?

Used as: filter, shown on the card. Data: on the listing, backfill cost low. Look for it as: Usually marketed directly ('cold water rated', 'environmentally sealed', 'cold water kit included').
Keyword match; default to not_rated_or_unstated when the listing doesn't mention cold water or environmental sealing.

### BCD weight integration

Whether the BCD has built-in, usually quick-release, pockets to carry lead weight, instead of needing a separate weight belt. Integrated weight pockets spread lead around the waist and let a diver drop it quickly in an emergency without reaching for a separate belt, which both jacket and wing-style BCDs commonly offer today. [10]

Values: integrated_weight_pockets, belt_only_or_unstated.
Question to ask: Should the BCD hold your weights, or a belt?

Used as: filter, shown on the card. Data: on the listing, backfill cost low. Look for it as: Usually stated directly ('weight-integrated', 'quick-release weight pockets').
Keyword match; default to integrated_weight_pockets for mid-range and above BCDs, since it's now near-standard on both jacket and wing styles.

### Mask internal volume

How much air space sits between the lens and the diver's face: low-volume (lens close to the face) or standard volume. A freediver or scuba diver spends part of every breath re-filling this space to equalize ear and mask pressure, so a low-volume mask wastes less air and feels less likely to press up under the nose on ascent. [1]

Values: low_volume, standard_volume.

Used as: filter, shown on the card. Data: needs research, backfill cost medium. Look for it as: Rarely a stated field; inferred from the model name ('Low Volume Freedive Mask') or category tag.
Needs the brand spec sheet or model-line knowledge; not reliably inferred from the title alone.
Chart: position on an equalizing-effort axis, low volume easiest.

### Fin blade stiffness

How much force it takes to bend the blade: soft, medium, or stiff. A stiffer blade moves more water per kick and tested more efficient for divers with strong legs, but a study found a softer, more flexible blade more economical for divers with less leg power — stiffness should match the diver, not the dive. [3]

Values: soft, medium, stiff.

Used as: filter, shown on the card. Data: needs research, backfill cost medium. Look for it as: Sometimes in the title ('Medium Flex'); otherwise only on the brand spec sheet.
Keyword match where stated; otherwise infer from the product line (travel/snorkel fins default to soft, technical scuba fins to stiff).
Chart: position on a thrust-per-kick-vs-leg-power-needed axis.

### Mask skirt material

What the mask's face-sealing skirt is made from: silicone, or rubber/PVC. Silicone stays flexible and seals well after years of sun and salt exposure, while rubber or PVC stiffen over time and are more likely to irritate skin; nearly every mask above entry price uses silicone. [1]

Values: silicone, rubber_or_pvc, unspecified.

Used as: filter, shown on the card. Data: on the listing, backfill cost low. Look for it as: Usually stated in the title or bullet points ('Silicone Skirt').
Title keyword match; default to silicone when unstated, since it's the dominant modern material.

### Snorkel mounting

Where the snorkel attaches relative to the mask strap: side-mounted (the recreational and scuba standard) or front-mounted and centered (used in competitive finswimming). A front-mounted snorkel reduces drag for straight-line swimming and finswimming racing, but it sits in the way of a regulator second stage, so scuba divers use a side mount almost exclusively. [2]

Values: side_mounted, front_mounted_centered.

Used as: filter, shown on the card. Data: derived from listing fields, backfill cost low. Look for it as: Usually clear from the product image or 'finswimming'/'centre snorkel' naming.
Default to side_mounted when unstated, since it's the standard; front-mounted is a small, clearly-labeled niche.

### Mask safety certification

Whether the mask is tested to a recognized diving-mask safety standard, such as the European EN 16805 or the American ANSI Z87.11, which require a tempered-glass lens. A mask that meets EN 16805 or ANSI Z87.11 has been tested for lens impact resistance and seal integrity; an uncertified mask may use ordinary glass or plastic that can crack or shatter against the face. [1] [6]

Values: en_16805_certified, ansi_z87_rated, uncertified_or_unstated.

Used as: filter, shown on the card. Data: needs research, backfill cost medium. Look for it as: Look for 'tempered glass', 'EN 16805', or 'ANSI Z87.11' in the description; rarely in the title.
Most mid-range and above masks state tempered glass; the specific standard number is usually only on the brand spec sheet.

### Snorkel safety certification

Whether the snorkel is tested to the European EN 1972 snorkel standard, which sets separate maximum tube length and dead-space volume limits for adult and smaller (child) users; full-face snorkel masks fall under the newer BS 8647 instead. EN 1972 caps how much stale, CO2-filled air sits in the tube waiting to be re-breathed; a tube longer or wider than the standard allows makes every breath work harder and build up more CO2. [2] [5] [11]

Values: en_1972_certified, bs_8647_full_face, uncertified_or_unstated.

Used as: filter, shown on the card. Data: needs research, backfill cost medium. Look for it as: Rarely stated on listings; look for 'EN 1972' or 'BS 8647' in the brand's spec PDF.
Default to en_1972_certified for standard open/dry snorkels from established brands; full-face snorkel masks need the BS 8647 flag.

### Decompression algorithm

Which decompression model the computer uses to calculate no-stop time and safe ascent limits: a Bühlmann-based model, a Reduced Gradient Bubble Model (RGBM), a Varying Permeability Model (VPM), or a brand's own blend. No algorithm has been shown to be clearly safer than the others when tuned to similar conservatism, but switching between a Bühlmann and an RGBM computer mid-trip can give noticeably different no-stop times for the same dive, which unsettles divers who don't expect it. [4]

Values: buhlmann_based, rgbm, vpm, proprietary_blend.

Used as: filter, shown on the card. Data: needs research, backfill cost medium. Look for it as: Rarely in the title; stated in the brand's spec sheet or manual, sometimes as 'ZHL-16C' or 'RGBM'.
Needs the brand spec sheet; mainstream recreational computers from most major brands use a Bühlmann variant unless marketed otherwise; code a Thalmann VVAL18-based computer as proprietary_blend.

### Regulator first-stage balance

Whether the first stage's mechanism keeps interstage pressure constant as tank pressure drops (balanced) or lets it vary with tank pressure (unbalanced). A balanced first stage breathes the same at 200 bar and at 50 bar, while an unbalanced one breathes slightly harder as the tank empties, which matters most on longer or deeper dives where tank pressure swings widely. [9]

Values: balanced, unbalanced.

Used as: filter, shown on the card. Data: needs research, backfill cost medium. Look for it as: Rarely in the title; stated in the brand's spec sheet as 'balanced first stage' or 'balanced piston/diaphragm'.
Needs the brand spec sheet; mid-range and above recreational regulators are usually balanced, entry-level unbalanced piston models are the exception.

### BCD lift capacity class

Whether the BCD is sized for standard single-tank recreational diving or built with extra lift for technical diving, such as a twin-tank set. A diver who moves from a single tank to a twinset or heavier technical gear needs a BC with more lift to float that extra weight at the surface, which is why wing-style BCs are commonly swapped for a higher-lift wing rather than resized. [10]

Values: standard_recreational, higher_lift_technical.

Used as: filter, shown on the card. Data: needs research, backfill cost medium. Look for it as: Lift is usually stated as a number on the brand spec sheet; treat any BC marketed for twin-tank or technical use as higher_lift_technical.
Needs the brand spec sheet; default to standard_recreational for single-tank jacket and wing BCDs without a technical/twinset claim.

## What you give up

- A stiffer paddle blade pushed more water per kick and was more efficient for divers with strong legs in testing, but a more flexible blade proved more economical for divers with lower leg power. [3] Board branch: "Stiffer blade, more thrust, more leg power needed".
- A dry-top valve keeps the tube from flooding on the surface, but the mechanism adds bulk and a float that can stick or add resistance that a plain open tube doesn't have. [2] Board branch: "Drier tube, more bulk and moving parts".
- A low-volume mask wastes less air each time a diver equalizes and resists pressing up under the nose on ascent, but sitting closer to the face narrows the side field of view compared with a standard or panoramic mask. [1] Board branch: "Easier equalizing, narrower side view".

## Words you will meet

- **Skirt**. The soft flange around a mask's lens that presses against the face to form a watertight seal.
- **Purge valve**. A one-way valve at the bottom of a snorkel tube that lets a diver blow trapped water out.
- **Dry-top valve**. A float-operated valve at the top of a snorkel that closes when a wave or submersion covers it.
- **Dead space**. The air inside a snorkel tube that holds a diver's own exhaled CO2 and must be cleared each breath.
- **Open-heel fin**. A fin with an adjustable strap and an open heel, usually worn over a boot.
- **Full-foot fin**. A fin that fits like a shoe directly over a bare foot, in a fixed size.
- **Split fin**. A fin blade with a lengthwise gap down the middle, marketed as giving propeller-like thrust.
- **Monofin**. A single wide blade worn on both feet together, used in freediving and finswimming.
- **Decompression algorithm**. The mathematical model a dive computer uses to track nitrogen loading and set safe ascent limits.
- **Bühlmann algorithm**. The most widely used decompression model, which tracks nitrogen in several body tissue compartments.
- **Air integration**. A dive computer feature that reads a diver's tank pressure, often wirelessly from a transmitter.
- **Internal volume**. The air space inside a mask between the lens and the face, which a diver re-breathes to equalize.
- **Tempered glass**. Heat-treated glass that resists shattering, required for mask lenses by diving-mask safety standards.
- **Anti-fog coating**. A factory coating on a lens that stops condensation from fogging the inside surface.
- **No-stop time**. How long a diver can stay at a depth before a decompression stop becomes required on ascent.
- **RGBM**. The Reduced Gradient Bubble Model, a decompression algorithm that factors in microbubble formation when setting ascent limits.
- **VPM**. The Varying Permeability Model, a decompression algorithm that models bubble nuclei growth to set conservative ascent limits.

## Where we made a call

- Split fin blades don't out-thrust a solid paddle blade once the split itself is blocked. (confidence medium). Sources disagree. Manufacturers market a propeller-like lift effect from the split; a cited swim-performance study found no significant change when the split was taped over. [3]

## References

1. [Diving mask](https://en.wikipedia.org/wiki/Diving_mask) (primer). Supports: Dual-lens masks sit closer to the face and hold less internal volume than single-lens masks; Mask skirts are silicone or synthetic rubber; silicone is the modern standard; Low internal volume is desirable for easier equalizing for freedivers and scuba divers; Mask lenses are tempered glass; EN 16805:2015 and ANSI Z87.11:1985 are cited standards.
2. [Snorkel (swimming)](https://en.wikipedia.org/wiki/Snorkel_(swimming)) (primer). Supports: EN 1972:2015 sets separate max tube length and dead-space volume limits by user size; Wet, dry (float valve), and semi-dry (splash deflector) snorkel top designs are described; A purge valve at the tube base drains water without removing the snorkel; Side-mounted is standard; front/center mounting is used in competitive finswimming; BS 8647:2024 is the first standard specific to full-face snorkel masks.
3. [Swim fin](https://en.wikipedia.org/wiki/Swim_fin) (primer). Supports: Full-foot fins fit like a shoe; open-heel fins use an adjustable strap over a boot; Blades are made of rubber, plastic, or fiberglass/carbon-fiber composite; Split fins claim propeller-like lift; a study found no change when the split was taped over; Stiffer fins suited divers with more leg power; flexible fins were more economical with less leg power.
4. [Dive computer](https://en.wikipedia.org/wiki/Dive_computer) (primer). Supports: Bühlmann, RGBM, VPM, and Thalmann are decompression algorithm families used in dive computers; No algorithm is shown to be clearly better; profiles converge when conservatism is matched; Some computers read tank pressure wirelessly from a transmitter on the regulator first stage.
5. [BS EN 1972:2015 - Diving equipment. Snorkels. Requirements and test methods](https://www.en-standard.eu/bs-en-1972-2015-diving-equipment-snorkels-requirements-and-test-methods/) (spec sheet). Supports: Standard specifies safety requirements for snorkels used by swimmers and divers at the surface.
6. [BS EN 16805:2015 - Diving equipment. Diving mask. Requirements and test methods](https://www.en-standard.eu/bs-en-16805-2015-diving-equipment-diving-mask-requirements-and-test-methods/) (spec sheet). Supports: Standard covers diving masks that protect the eyes during underwater breathing; excludes full-face masks.
7. [Suunto Dive Computers](https://www.suunto.com/en-us/Products/Dive-Computers/) (spec sheet). Supports: Brand markets viewing tank pressure directly on the dive computer as an air-integration feature.
8. [Speedo Goggles](https://www.speedo.com/us/en/swim/goggles/c/goggles) (spec sheet). Supports: Brand categorizes goggle lenses as coloured, anti-fog, polarised, and mirrored; Brand categorizes goggles by type as training, racing, and recreation.
9. [Diving regulator](https://en.wikipedia.org/wiki/Diving_regulator) (primer). Supports: Balanced first stages hold a constant interstage pressure as tank pressure drops; unbalanced ones vary; CGA 850 yoke (A-clamp/international) and DIN screw-in are the two standard cylinder valve connections; Cold water kits or sealed designs reduce the risk of regulator free-flow and internal icing; A secondary demand valve (octopus) supplies an out-of-air buddy from the same first stage.
10. [Choosing a BCD: Jacket or Wing](https://scubadiverlife.com/choosing-bcd-jacket-wing/) (specialist review). Supports: Jacket-style BCDs wrap the torso and hold a diver upright; wing BCDs put the air cell on the back only; Wing-style BCs can be swapped for a larger wing for greater lift on technical dives like a twinset; Both jacket and wing-style BCDs commonly use weight pockets instead of a separate weight belt.
11. [BS 8647:2024 - Respiratory equipment. Full-face snorkel masks. Specification](https://www.en-standard.eu/bs-8647-2024-respiratory-equipment-full-face-snorkel-masks-specification/) (spec sheet). Supports: Standard title confirms BS 8647:2024 covers full-face snorkel mask respiratory equipment specification.