User blog comment:Rocket.knight.777/Opinions on a possible fic/@comment-4080028-20140821055338/@comment-3387274-20140821140339

Permenant effects of Devil fruit still work underwater, but the user cannot activate any controllable powers they have, nor can they control them. This is in adition to being paralysed/weakened.

Basically, if the user is able to fire beams, they would not be able to underwater. if the user had been turned into stone, however, they would still be stone even after being submerged, assuming the power was a permenant thing they couldn't turn off.

Due to the water weakness the current ruling is that a water manipulation power cannot be allowed whether or not it is underpowered or overpowered, so I'm afraid you will need to come up with a different fruit if you want to do that.