There was a scene for Atlantica in the trailer?
Hmm, I guess I never really payed attention.
If he was a fish in that world, then it would ruin the gameplay since he can swim over ledges.. So what's the point of that? And they would need to add more extra combos to it in battle.. So yeah.. there's the reason. But I guess it WOULD be weird to see everything swimming around and Sora is just standing there with infinite breathing cheats.