Even if a fishman was forcefed a DF he or she would probably not be in Fishman Island because it's, you know, under water.
And the only place to get a ship coated is on Saobody, which, you know, enslaves fishmen on sight.
What if the new crew member ate the Swim Swim fruit that grants the ability to swim, nullifying the devil fruits power and essentially doing nothing other than providing us with some good laughs. That won't happen but it would be funny. Wait, is a Swim Swim DF even possible?