When it comes to underwater photographers, there are two types: snapshooters and artísts (that's they way they say it). The former are those who simply want fun photos to show and share. The latter want quality photos that could pass as art or publication-worthy.
Snapshooters do well with compact, general-purpose point-and-shoot cameras. Frame the shot and push the button. These systems work best in clear water and only require a moderate investment.
Serious photographers opt for more sophisticated cameras with interchangeable lenses. The camera may have automatic functions, but the photographer needs to be able to adjust or completely override them. These systems use large, separate submersible strobes with TTL (Through The Lens) strobe control. These systems represent a more sizable investment.