@Mr_MustacheIf you want to barf, play Descent (or its N64 clone, Forsaken).
I'm here and there on the underwater fights in Monster Hunter (which were removed in MH4, btw). The idea is cool, and it IS genuinely different, but it's not like it's actually fun to swim. It's slow(er) and clunky(ier). I was talking more about movement options that make it MORE fun to fool around in the game world.