I feel like the only "fish" Devil Fruits we'll ever see are those species that are at least partially amphibious. Because...you know...the whole "Devil Fruit Users are weak to water" thing. It'd be kind of lame if you got a cool Shark Devil Fruit or something and couldn't really use it, because it can't breathe out of water, and the User can't go INTO water without seizing up.
It's probably why we don't see too many "liquid" Logias. It kind of raises weird questions. Like, if you turned your whole body into...milk or something, would that trigger the weakness to water or not? Because milk is mostly water. Could a User be weak to their own Devil Fruit?
I feel like Oda's going to mostly side-step these more awkward questions, by just never really addressing it, directly. At least, maybe until after he explains how Devil Fruits were first created in more detail. He could give us some sort of lore reason for it.