When you drive a car under water, after few seconds you are ejected automatically and can swim until your character cannot hold breath anymore (best effect in any game I've seen, no health bar but vanishing strength due to leak of oxygen, with a nice tunnel vision effect!).
I think you should remain in the car and must eject manually, like on land.
When a helicopter goes under water (slowly), you die instantly. It should behave just like the car as I suggested above.