Wetsuit

A neoprene suit that insulates a diver by trapping a thin layer of water against the skin, worn in a wide range of water temperatures.

A wetsuit keeps you warm underwater by letting a thin film of water in and then holding it still against your skin, where your body heats it. The foam neoprene itself is full of tiny gas bubbles that insulate you from the surrounding water. Water pulls heat from the body far faster than air does, which is why even tropical divers usually wear at least a thin suit.

Thickness is matched to water temperature: a 3 mm suit suits most tropical diving around 24 to 29 C, 5 mm covers temperate water, and 7 mm suits, often with a hood, handle cold water in the 10 to 18 C range. Fit matters as much as thickness, because a loose suit constantly flushes warm water out and cold water in.

Neoprene compresses as you descend, so a wetsuit loses both warmth and buoyancy at depth, which is why your weighting and BCD adjustments change through the dive. Logging the suit you wore and the water temperature makes it much easier to nail your weighting on the next trip.