●Open the window as fast as possible — before you hit the water, if you can, or immediately afterward.The next three steps are described at the Washington Post.
●Stay still, with your seat belt on, until the water in the car goes up to your chin. Then take several slow, deep breaths and hold one.
●Do not try to open the door until the water has stopped flooding into the car. Initially, the water outside will put pressure on the door of up to 600 pounds a square inch, meaning you won’t be able to open it from the inside. The pressure inside and outside the car should equalize about the time you start holding your breath.
p.s. - won't some cars float for a reasonable length of time? I think my old VW Beetle was supposed to float, and I remember seeing floating cars in videos of the Japanese tsunami.