I believe that it's much simpler, the sea just keeps them in the basic form of power, luffy's case it's not that streachy human, ace's case would be,not flames, Choppers case, a human/reindeer, thats why when he was thrown as rampaging chopper he came out as human/reindeer chopper, every devil fruit user has a sort of standby face, where if thrown in the water, he is trapped there, and their powers are greatly reduced, not taken away. sea stone rock, takes the power away as long as they are in contact.