I always found it interesting how when fish swim they move their bodies first from the head and then to the rest of the body, Occulus Rift would be the best way to swim like a fish! Controls for the game will be a challenge as getting the correct head movements calibrated for fun gameplay will be tricky, but a worthy challenge and a worthwhile game. The game itself could really be a whole number of things. Swim in a fish tank and try to eat food before other fish (players). Survive the pet store, see how long you can last in a fish tank before the net gets you.
My personal favorite idea is play as a fish in an open ocean and try to survive fishing trawlers while swimming through a dying landscape. The intent of this version is to raise awareness for overfishing.
Beautiful fish courtesy of Creative Commons