The first thing you need to do in order to get rid of a car underwater is to free yourself from the seat belt wrapped around your waist. Remember that you have very little time to get out of the car so immediately cut through the seat belt if you fail to unbuckle it in a couple of attempts. Use a rescue blade or anything sharp to cut through the belt.
Keep in mind that you have to equalise water pressure inside and outside the car in order to get out of it. This can only be accomplished when water starts flowing into the car. Like it or not, you will have to let water into the car to equalise pressure. Roll down the car window after taking a deep breath. It is possible that you may not be able to roll down the car window because the water pressure on the window was too great. If that is the case, you will have to break the car window with an emergency hammer or any other heavy object. Water will rush into the car. Simply swim out of the window if you are slim enough to fit.
If you are too bulky to swim through the car window, you will have to wait because you will not be able to open the door as water pressure out side the car is still greater than inside the car. Take another deep breath and wait until water completely fills the car. At this point, you can open the door easily and swim to safety.