>>>>.Note it is very hard to open a car door against the pressure of all that water and modern cars electic windows would be shorted out.<<<<
In fact trying to open a door before a car is submerged makes the biggest chance to die. A vehicle tends to heel on a side of your opened door to sink faster and land on a bottom right above you.
The best idea is to leave a car through your opened window but it is not always possible because most vehicles are sinking nose-down.
If you haven’t managed and sinking with locked windows, you have to undress to fold seats for more room and grab any floating items you have inside (even an empty plastic bottles).
As soon as your car is full of water and there are right enough space just to make a few breaths, get out (doors are opening easily at the time).