I love that Oda actually gave a fishman a devils fruit. That's genius.
The bubble thing, I dunno... Luffy had a bubble that only surrounded his upper half, and then his legs were just coated otherwise, maybe it's just a design oversight. If a fishman with a devil's fruit is exposed to sea water, what happens, exactly? Do they just sink to the ocean floor and lay there motionless? Can they still breathe?
A lot of the sets in this arc is kind of confusing. It's hard to tell half the time whether they're underwater or not. Neptune looks like he's floating around, but obviously the strawhats shouldn't be able to just stand there if the palace was filled with water.