A wetsuit is a bodysuit made of neoprene or other rubber, and are usually (but not always) onepiece. They are most commonly used in scuba diving, surfing, and triathlon racing.
Most versions work by allowing a small amount of water inside, that is then heated by body warmth. Such a setup requires the suit to be a tight fit by necessity. The suits can also protect from jellyfish stings and from ultraviolet sunburns.