well there are about 3 possibilities.
1. Yes there might be a water fruit user. However he would probably only be immune to the water he produces.(so he would have the same weakness if he falls in water(other than his own), but he would be able to control water.)
2. Yes, but he would just be a vegetable(unable to move)
3. There might be a water fruit user, who is just somehow not weakened by water.(most unlikely)
It is possible a water df can exist. I highly doubt we will ever see one, even if it is possible.