THE MOST IMPORTANT thing to do is DO NOT PANIC--REMAIN CALM, COOL, SO AS TO KEEP YOUR WITS ABOUT YOU, IN ORDER TO BE ABLE TO THINK AND ACT PROPERLY.
With heavy [high pressure] water pressing against the door [gate] from the outside, and low pressure air on the inside, for all practical purposes, it is impossible to open a gate [door].
A person in this situation, has a few choices for safely getting out of the car.
1. Be patient and WAIT until the car fills with water [thereby "balancing" the pressure on the inside with the outside], allowing the door to be opened. You can continue to breath by keeping your nose and mouth in the air "bubble" near the top of the passenger compartment.
2. Immediately break the window glass out, and get out while the car is still floating with the bottom of the window still above the surface of the water. Once the glass is broken, the car will fill with water MUCH FASTER than with the windows intact. This means that once you have broken the window, you will need to be ready to get out fast!
3. Wait until the car is underwater and almost full of water, with just a little airspace near the top [you can breath in this area], and THEN break out the window, and swim out to the surface.
Again, if this happens to you, DO NOT PANIC. Remain CALM, and follow the procedure [s] above, which will allow you to save your life, and those of your companions in the car.