The term 'underwater swimmer' can be described by two words: aquanaut (8 letters) and FROGMAN (7 letters). An aquanaut is a person who lives and works underwater for extended periods of time, typically in a research or exploration capacity. A FROGMAN is a trained diver who is used for military or police operations underwater. Both aquanauts and frogmen are highly skilled swimmers who are able to operate in extreme underwater conditions.