An underwater camera refers to any camera designed to work fully submerged in water. This means no separately purchased housing is needed. Some require a little assembly whereas others, like the Nikon W300 (our #3 pick) come in one piece, ready to go for a swim. An action camera is a specialized underwater friendly camera in the mold of the Go Pro. They’re typically designed to be mounted somewhere and record a very wide field of view, rather than have you aim and compose each shot.