In the One Piece universe, Zoan Devil fruits give people animal-like abilities. Now granted, there are already amphibious mere-folk in this manga, but that's not to say there isn't (or at least, I have not yet seen) any Devil fruit that can give someone fish-like abilities.
If that's the case, would a Zoan fish Devil Fruit (say, a Fin Fin-fruit) that gave someone adapted aquatic abilities still be bound by the seawater weakness?
There are instances where this rule has been somewhat circumvented.... for instance, Brook can run on sea water without any ill effects, and his immortality means he can't drown.... however, it's uncertain if he were to sink into sea water if that same immortality would be negated and he would thus die permanently. It stands to reason, though, that if a person is a "fish person" then swimming in sea water would be a natural consequence.
Are there any instances of this occurring, or can anyone elaborate if this may not be the case, i.e., even given a new "fish-like" body, they would still be subject to the sea water-weakness rule?