This is a skill you most likely won't ever need, but if you do ever find yourself in this situation, you better believe you're going to wish you would have learned. There's a surprising amount of science involved with this death. The doors won't want to open because of the pressure, and as the car slowly fills with water, you'll feel trapped. Truly terrifying. As if it isn't scary enough to have driven your car into water.
I keep a knife in my car that can cut through seatbelt and smash through a window if need be. The only thing is that I don't know where it is at the moment because it keeps rolling around in the car. I'd have to spend eight minutes trying to find it as my car filled with water.