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The Core Difference
A wetsuit works by trapping a thin layer of water against your skin, which your body heats. The neoprene insulates this water layer. As long as the layer stays thin and relatively still, you stay warm. As water exchanges — due to a poor seal at the neck, wrists, or ankles — you lose warmth rapidly.
A drysuit keeps you dry. A waterproof shell (either compressed neoprene or a membrane material) seals at the wrist and neck with latex or neoprene seals. You wear thermal undergarments beneath it. Air inside the suit provides insulation — and because you control that air volume, a drysuit also functions as a buoyancy device.
The fundamental divide: wetsuits are simpler, cheaper, and require no special training. Drysuits are warmer, require a specialty certification, and demand more attention during the dive.
Wetsuit Thickness Guide
Neoprene thickness is measured in millimetres. Thicker = warmer, but also stiffer and more buoyant (requiring more lead weight to offset).
- 0.5–1mm (lycra/skin suit): Tropical water above 29°C; sun protection only
- 3mm full suit: 24–29°C; the standard 'warm water' wetsuit
- 5mm full suit: 18–24°C; suitable for Mediterranean, Caribbean, and tropical liveaboards in cooler months
- 7mm full suit: 14–18°C; UK summer, Pacific Northwest, deep thermoclines in tropical destinations
- Two-piece systems (7mm + 5mm jacket): 10–14°C; stacking suits for redundant insulation
Fit is critical. A wetsuit that gaps at the neck loses its thermal function immediately — water flushes through continuously and the suit becomes a cold pump. Try wetsuits on with the same undergarments (if any) you plan to dive in.
Semi-Dry Suits
A semi-dry is a wetsuit — typically 5–7mm — with improved seals at the wrist, ankle, and neck designed to dramatically reduce water exchange. They are warmer than a standard wetsuit in the same thickness without the complexity of a drysuit. For divers in 14–18°C water who are not ready to invest in a drysuit, a semi-dry is a meaningful intermediate step.
When to Move to a Drysuit
The answer varies by diver, but general guidelines:
- Below 14°C: A drysuit becomes strongly advisable for dives longer than 20–30 minutes
- Cold water technical diving: Drysuits are almost universal
- Long multi-dive days in 16–18°C water: A drysuit maintains thermal comfort across 4–5 dives per day when a wetsuit's insulation saturates with water and loses effectiveness
Drysuit Types
Compressed neoprene drysuits are made from thick neoprene (3–5mm), which provides inherent thermal insulation even without undergarments. Heavier, less packable, warmer for their undergarment requirements. Common for UK and Nordic diving.
Membrane drysuits (trilaminate or bilaminate) are shells of waterproof laminated fabric with no inherent insulation. All thermal protection comes from undergarments. Lighter, more packable, flexible across a wider temperature range (change the undergarments, change the temperature range). Preferred by technical divers who need to be able to adapt rapidly.
Drysuit Training
Diving a drysuit without training is dangerous. The air inside the suit must be managed during descent and ascent: too much air in the suit causes an uncontrolled buoyant ascent; too little causes a 'squeeze' as the suit compresses. The PADI/BSAC Drysuit Specialty course teaches buoyancy management with the suit inflator and the suit dump valve — the two controls that do not exist in wetsuit diving.
Undergarment Selection
Drysuit undergarments range from thin fleece one-pieces (for 15–18°C) to expedition-grade thinsulate systems with separate top and bib (for sub-10°C or extreme cold). The right undergarment matters as much as the drysuit itself — a good drysuit with inadequate undergarments will leave you cold.
For most recreational drysuit divers diving at 10–15°C, a mid-weight undersuit (250–400g/m² thinsulate or equivalent) is appropriate. UK and Scandinavian divers frequently use heavy undergarments year-round.