This is one of the most dangerous situations you can encounter because it takes away the control from you. You are not longer on the road and the car is filling with water. What should you do in this situation?
Let’s presume that you drove your car in water but you haven’t suffered serious injuries. First of all, DON’T PANIC! See if you can remove the seatbelt. If not, take your Swiss army knife and cut it. Do the same for the others passengers. Calmly, try to breath as much air possible while the car is filling with water. You will see that you can’t open the doors as long as it is air in the car so try not to spend energy doing so. Instead, breath as calmly as possible and when the car is filled with water, open the door and swim out. If the door was jammed, it would be useful to have a special hammer to break the window and swim through it. Don’t try to save stuff from the car. It is more important to save yourself and the passengers.
If something like this happens, the most important thing is to not to panic. By panicking, you consume much more air and you don’t think straight. Tell the other passengers to stay calm. I know it appears that you don’t do anything, but wait for the car to fill with water and after, swim out.