If you can open the door, do so, and it's worth a try. Best to get out quick if you can. But as I understand it, the weight of water soon makes it impossible even if the car is only partially submerged. The main thing is to understand why, and to realise that the door will open again when the car has filled with enough water to reduce the pressure difference. And until then you will have the air in the car to breath. So there's no need to panic just because the door won't open. People who don't know this may think they are trapped forever, and give up.