I'm with gvr4ever on this. Luckily, I participated in a water safety training course for open/deep water survival (don't ask). The course also include training in egression techniques from different vehicles (auto, boat, airplane). They actually have this huge tank that is used to simulate the exercise. They strap you in - you got for a ride down a slide into the water in complete darkness to disorient you (they flip the vehcile body over a couple of times to make sure). Then you have to find your way out safely (safety divers on hand to make sure you do make it back safe).
An emergency tool might be a good investment - if you can find it and use it during such an accident. No amount of safety tools or devices can save you if you start to freak out or are badly injured in a car accident. That being the case - there are many tools out there that clip onto your belt or get affixed to the dash, that can cut through seltbelt webbing and break glass. A spring loaded center punch is also perfect for shattering side glass on cars.
First thing - stay calm.
Second - undo your seltbelt (actually the number one killer here). Documented cases have seen people with the window down or the door open, but the person still drown becuase they could not undo the seltbelt.
Third - help others in the car if you can.
Fourth - get out ASAP.
Military likes acronyms - they call this P.O.G.O. (PUSH the seatbelt button, OPEN the window or door, GET OUT of the car).
If the water is 15-20 feet - there is a good chance that the car will flip onto its roof. Not a good thing, as underwater - it is very difficult to orient ones self - throw in injuries, darkness, etc. - a bad proposition for anyone.
The best method is to get out ASAP - the car will float for a little while before it sinks, some cars can float for as long as two minutes before they take on enough to start sinking. You can power down the windows and escape from the window - do not open the doors as it will be either difficult for most people to do or cause the car to rotate in that direction into the water.
Once the car takes on water - it will sink VERY quickly. You will not get an air pocket like they show on TV all the time. If you could not get out before the car sinks - wait until the water gets to neck level (happens really quickly), take a final breath (seatbelt should already have been undone as soon as the car hit the water), open the door (if damage in the accident) break open the window, then escape in the direction of air bubbles (you should be exhaling air as you ascend to protect your lungs and to give you a direction to swim - bubbles traveling upwards).
If there is a strong current, if the water temperature is very cold, or if the body of water is iced over - it is imperative to get out before the car sinks.