When a devil fruit user hits the water, their physical strength is sapped out of them and they can't control their powers (for example, Luffy could still stretch when he was stuck at the bottom of Arlong park, but he could not control that stretching).
May I remind you guys rubber is buyant in water. It floats. If the nature of a person's devil fruit powers could stop them from sinking, Luffy would not sink.
Aojo Kiji is probably the only one who has some defense against the sea. If he were to freeze it before falling in, he'd be safe. However, if fell hell in, he'd sink just like everone else.