A couple of people seem to be wondering if the way Camel got out is true. And it is. Though, it's something you should use as a last resort. The best way to get out of a car submerging in water is first get out before it can completely submerge in the water and start filling in the car. The second is to break the window. Not with a hammer, but something that can focus all your effort into a single point. And finally, of course, is waiting for the car to fully fill up so the pressure can equalize from both inside and outside the car.
Mythbusters actually tested out on ways to get out of cars being submerged in water, so if you wish to find out more, you can watch them.