It's not that they sink like a stone, but they lose all strength and are immobilized, which means they can't swim at all. Even if only partially submerged they lose a lot of strength. But that's beside the point.
The answer would have to be pure speculation at this point. And I would guess that they do float. DFs sinking to the ocean floor would mean they would completely disappear, since no one ate them and died to let it respawn.