And just for clarification, DF users don't lose their powers in water, they lose control of them.
What do I mean?
Way back when Luffy fought Arlong and got thrown into the sea, his body (neck) was still rubber, and that's how he was kept from drowning.
So if a ship had a DF, and was weakened by water, it would still have acces to that power. Though it would probably be not used to it's fullest.