Dr. Gordon Giesbrecht, a thermophysiologist from the University of Manitoba, has studied over 100 car submersions. He says you have 30-60 seconds to get out of a sinking car, and you don't want to wait until the car is completely underwater.
1. Don't touch your cell phone. Some people have the impression a car will float long enough for them to call 911. A car will float for a minute, and you need to escape in that first minute. Beyond that, you'll have very little chance of getting out.
2. Release your seatbelt. In panic mode, many people forget this step.
3. Open or break a window. Don't bother with the door. If the electric windows short out, use a small hammer or small punch. And if you don't carry one of those, buy one soon. Dr. Giesbrecht keeps one mounted on his dash.
4. Grab the kids. If you have younger passengers, unbuckle them starting with the oldest first.
5. Get out, pushing the kids ahead of you. Dr. Giesbrecht found that it was possible to escape through a window, even with a torrent of water flowing in.
Sources:
ABC News, "What to Do: Car Sinking, Seconds to React" from 3/25/2013
The Globe And Mail, "How To Escape A Sinking Car" from 8/8/2010