Brangdon,
There was a show on tv where the little guy from Top Gear tested to see if the best way to get out of a car was to wait for it to sink to the bottom and than have the pressure equalize. He tried it and if I remember right was unable to open the door even though the water pressure was more or less even. So, from what thye said, the best option seemed to be; get out as soon as you hit the water or break a window and get out as fast as you can.