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Scuba Diving: What Size Dry Bag Do You Need?

What must stay dry versus what is already wet

Draw the line before you pack rather than after the first dive. Personal items that cannot tolerate salt water are few but unforgiving: certification card, passport, cash, phone, medication, dry clothing for the ride home and possibly a camera control unit. Everything that already comes out of the sea wet — fins, mask, regulator, BCD, boots, wetsuit — should never enter the same bag as those items.

That separation is more important on a dive boat than on most other craft because salt water is continuously present. Decks are hosed down between dives, gear is stacked still dripping, and the spray pattern over a bow at 18 knots hits every bag on board. Anything personal stays sealed and preferably below deck or inside a bench locker rather than on an open rail.

There is a second reason for keeping them apart: weight and volume behave differently. Wet gear gets heavier through the day as it absorbs water and accumulates sand, while dry kit stays constant. Putting both in one container means the dry kit is exposed to a growing pool of salt water for six hours, which is exactly how people lose a phone on the second dive of the day.

Size by dive type, not by brand range

Your dive profile decides the size before anything else does. A shore diver changes in a car park and carries kit 200 m to the entry point, so everything personal has to fit one bag that also survives being dropped on wet rock. A day-boat diver has a bench locker and needs less carry comfort but more sun protection. A liveaboard diver is living out of bags for seven days and needs volume plus organisation.

Read the third column carefully, because it is where people misjudge. A single diver genuinely needs very little capacity; the leap happens when photography or multi-day living enters the picture. If you dive twice a year and never take a camera, a 10L bag is a better purchase than a 40L one, both because it costs less and because it fills a small space on a crowded deck.

Consider shared loads too. Two divers sharing a boat share the splash, so if both bring personal dry bags, place them together in the driest available locker rather than splitting them across the deck. Redundancy is useful on a kayak, where a single failure is survivable only if gear is split; on a dive boat one protected location protects everything.

The 10L versus 20L decision on a day boat

Ten litres holds a compact towel, a dry shirt, shorts, logbook, card holder and phone — about 6.5 litres packed, leaving room to roll. It fits under a bench, in a small locker or at your feet, and it is light enough that nobody objects to it being carried on board. For most warm-water day boats this is the honest answer.

Twenty litres becomes necessary when three conditions appear together: you change fully rather than partially between dives, you carry a camera control unit or a second set of dry clothing, or the boat has no protected storage at all. In those cases the extra capacity buys real protection, since a bag that must hold everything can be rolled three times and left closed all day.

Breaking the load into roles

The pattern that works reliably is two small bags rather than one large one. The first holds what you touch between dives: water, snack, sunscreen, logbook and certification card. The second holds what you only need at the end of the day: dry clothing, shoes, a towel and anything electronic that stays switched off until you are ashore.

That split gives you a bag you can open during a surface interval in driving spray without exposing everything else, which is the same logic used for paddleboard day kit. On a rolling boat, a single large bag gets opened more often than necessary, and each opening is another minute in which salt water can reach something it should not.

Notice that the wet-gear container is deliberately not a dry bag. A sealed roll-top holding a wet wetsuit for eight hours produces condensation, odour and a mildew risk by the second day, while an open mesh crate lets everything drip and air. Divers who insist on sealing wet kit are the ones asking about smell removal by day three.

Boat deck realities: spray, sun and space

A dive boat is a harder environment for a bag than a kayak is. Spray arrives from bow and beam, the deck is hosed with salt water between dives, and every cubic metre of usable space is contested. Check the sea state forecast through NOAA the night before, because a 15 to 20 knot run changes where you can safely leave anything. Any bag left on an open rail will be soaked outside within twenty minutes of getting under way, regardless of how good the closure is.

So the first habit is positional: put personal bags in the lowest, most sheltered storage available, usually a bench locker or below deck, and never on the deck itself. If there is no storage, wedge bags together against a bulkhead rather than leaving them individual targets, and keep them away from scuppers where rinse water and fish wash collect.

Sun brings one more consideration that belongs here rather than in the care section: what you put on your skin ends up in the water. Sunscreen residue transferred from hands to closures then to rinse water is a small but genuine contribution to what a reef absorbs, and guidance on reef-safe choices is published by NOAA. Wash hands before handling bags on board and the amount transferred drops sharply.

Heat remains the second deck reality even with a reef-safe routine. A black bag on a tropical deck can reach about 60 °C at the surface, which both softens the film and accelerates ultraviolet ageing, so choose a mid-tone colourway where you can and keep bags shaded rather than in full sun. Three weeks a year in the tropics ages a bag about as much as three years of occasional weekend use, which is worth remembering when comparing prices across otherwise identical bags.

Certification card, passport and cash

These are small, high-value, impossible to replace quickly, and routinely ruined by nothing more than a wet shorts pocket. Keep them together in one 1L to 2L roll-top that stays on your person or in a sealed pocket, rather than distributed across three bags where one of them is likely to be opened at the wrong moment.

Print a paper backup of anything that can be printed, and keep it in the same pouch. A phone showing a certification card is fine until the battery gives out on day four, at which point a paper copy is worth more than the phone itself. Screenshots stored offline help where there is no network anyway.

There is a practical trick worth adopting from longer trips: pack one set of dry clothing and valuables in a bag that never leaves your side, even when other luggage is checked or stowed. Our passport protection notes cover this in a travel context, and the same logic applies on the dive boat where shore leave follows the last dive.

Between-dive rinse and closure care

A roll-top closure fails far more often from grit than from pressure. On a dive boat there is always salt crystals and sand around, and one grain in the fold creates a path for water to wick inward over an hour. Rinse the fold line with fresh water from the boat's rinse tank before every roll, then run a thumb along it to check nothing is trapped.

Water inside the bag is then almost always chloride-contaminated rather than merely damp, and that is the start of corrosion on anything metal inside. If you find moisture on the inside surface after a day afloat, rinse the whole bag with fresh water at ambient temperature rather than just drying it out; drying leaves the salt behind.

Cameras, housings and electronics

Photography changes the plan more than anything else in diving. A compact housing and its control unit want a sealed dry container that opens only in a dry area, and the ideal is for the camera to go into the bag dry and come out of it dry rather than travelling damp between dives. Rinse the housing outside the bag, never over it, and treat the O-ring sealing surfaces as clean-room components rather than dive gear.

Size accordingly. A compact setup adds roughly 4 to 6 litres including padding, and a housed DSLR or mirrorless rig adds 10 to 14 litres, which pushes a day-boat diver from a 20L up to a 20L plus a 5L pouch for batteries and cards. Pack batteries in a small internal roll-top, because contacts corrode faster than anything else in a salt environment.

Condensation is the specific risk for camera kit: a housing sealed while warm and humid, then carried into an air-conditioned cabin, collects moisture inside exactly where you least want it. Close electronics bags in the driest available place, keep a silica sachet inside each one, and change that sachet every couple of days on a trip.

Flying home with damp dive gear

Airlines are the practical constraint on the last day. Wet scuba gear is heavy, checked pieces are weighed, and nobody wants five kilograms of damp wetsuit in a suit. Divers handle this by splitting into three: fully dry and clean inside the check-in piece, damp neoprene in a dedicated open bag, and anything valuable or electronic carried on.

Roll-tops earn their place at this point, because a sealed bag prevents a damp item from soaking everything else in transit. Roll twice rather than three times to avoid sweating inside a sealed container, and pack any still-damp item with a spare towel rather than directly against dry clothing. Our advice on rinsing before travel applies: do it the evening before, not in the morning.

There is also the matter of air pressure in flight. A tightly sealed roll-top holding warm air expands as cabin pressure falls and can push open at the closure, so reduce the volume slightly before sealing or leave the buckle lightly tensioned. Nothing dramatic happens to the bag, but damp contents have a habit of escaping at exactly the wrong moment.

Liveaboard: living out of bags for seven days

On a liveaboard the bag is your cabin furniture. You need volume, but more importantly you need a consistent location for each category, because you will repack the same bag four times in a week and everything that is not in a fixed place ends up lost in a small cabin.

The arrangement that holds up well is a 40L bag for clothing and spare items kept in the cabin, a 20L day bag that travels to the dive deck every morning, and two or three pouches for cables, medication and documents. Keep the day bag packed the night before; the 05:30 call is the point where organisation usually collapses.

Humidity is the liveaboard factor people underestimate. Cabins run air-conditioned and dry, then doors open to 85 per cent external humidity four times a day, and anything porous absorbs the difference. Keep closures sealed between uses rather than leaving bags open overnight, even though airing is good practice back on land.

Buoyancy: what a dry bag is not

Every diver eventually looks at a sealed dry bag and thinks about lift. It is worth being precise: a 20L bag with most of its air pressed out displaces very little, and its buoyancy disappears the moment the roll opens under load. It is not a lift bag, it is not a surface marker, and it is not a personal flotation device.

The genuine use is as emergency support for small items — keeping a camera or a torch afloat if it goes over the side in shallow water. For that, roll it three times, leave plenty of air inside, and tether it rather than relying on the buoyancy alone. Do not trust it with anything you could not afford to lose.

There is also a legal and safety dimension in some jurisdictions, where carrying equipment that resembles a buoyancy aid without certification is discouraged. Our guide to when a dry bag is not a life jacket sets out the boundary clearly, and the same conclusion applies to any dive application you might imagine for it.

After the salt: routine that decides service life

Salt damage accumulates quietly. Crystals form inside a fold as moisture evaporates, and those crystals then abrade the film every time the bag is rolled, so the damage is mechanical as well as chemical. Rinsing while everything is still damp is far more effective than trying to clean dried salt out afterwards.

Fresh water at ambient temperature, a soft cloth along the closure line, and a full dry in shade before storage is the whole routine, and it takes about eight minutes. Add a monthly inspection of welded corners and of the area around the D-rings, which is where fatigue appears first on a bag used every weekend.

1. Empty the bag completely and shake out sand before rinsing anything.
2. Rinse with fresh water at ambient temperature; never use a hot hose.
3. Wipe the closure line with a soft cloth and check for salt crystals.
4. Work buckles and cam levers through their full travel while still wet.
5. Dry open in shade, then store loosely rolled with the buckle unclipped.

Then think about where it lives between trips. A cool cupboard beats a garage, and loosely rolled beats folded flat, because a stored fold becomes a permanent crease on a warm day. Our longer-form notes on saltwater service life suggest these habits extend a coastal bag from around three seasons to five or more.

Choosing the one bag to buy first

If you are buying a single bag for diving and nothing else, choose 20L in a mid-weight construction with welded seams and reinforced hard points, in a colour that hides deck grime. Twenty litres covers a day boat with room to roll, still fits a flight bag, and is large enough to double as carry-on for a dive holiday.

Add a 5L pouch in the same order for documents, cards and batteries, because that combination handles every situation in this guide from shore diving to photography. Then decide whether you need anything larger only after a season of real trips, rather than buying a range that turns out to be mostly unused volume.

Compare sizes against the roll-top range before committing, and check that whichever bag you choose shares a closure geometry with any others you own so one rolling habit covers all of them. Divers buying for a club or a charter operation usually standardise on two sizes, which simplifies crewing and leaves fewer bags unaccounted for at the end of a week.