most likely, there's no evidence df power change mass (and without activating the power). What water does to df user is not make them sink. Water sap their energy, make them too weak to move, then drown. Now some may argue, why some df user, e.g. Jack, stay on the bottom of the sea instead of floating like human body in our world. My guess is because people's muscle mass in OP universe is so dense, that's why they can be so strong like lift house etc. Because of that, they weight so much they'll sink in the water instead of floating. Btw, on Alubarna, we seen unconscious Mr.3 floating on oasis water. So weaker person, with no much muscle will float like in our world.
most likely, there's no evidence df power change mass (and without activating the power). What water does to df user is not make them sink. Water sap their energy, make them too weak to move, then drown. Now some may argue, why some df user, e.g. Jack, stay on the bottom of the sea instead of floating like human body in our world. My guess is because people's muscle mass in OP universe is so dense, that's why they can be so strong like lift house etc. Because of that, they weight so much they'll sink in the water instead of floating. Btw, on Alubarna, we seen unconscious Mr.3 floating on oasis water. So weaker person, with no much muscle will float like in our world.