I hate Pokémon that are obviously meant to be used in water and in water alone.
I fucking hate seeing floating fish in pokémon all the time. Either battle in the water or don't capture a fucking fish or something with the shape of a fish.
So about your dolphin, fuck that. What is it gonna do? Immediately strand and flail around like a magikarp upon leaving the pokéball in the middle of the stadium? No. You want a magically floating dolphin. Fuck you.