Not being able to attack while underwater? That's a terrible idea. It's a power up, you don't need any trade-offs.
Not necessary a power-up and even so adding a trade-off usually helps balance things out. As for not being able to attack, considering that game seems to be favoring infiltration over fighting, I don't really see an issue there, though it'd depend on how exactly the whole underwater part play out.
I guess it could work in a Donkey Kong Country way. In fact, now that I think about it, the thing about being able to attack while in human form but not when in fish form would be the complete opposite of that game.