This was in my issue of Car and Driver for this month and I thought it would be helpful to those who might find themselves in this situation.
The article is by Tony Quiroga.
Give credit where credit is due.
Step One: Don’t panic. Unbuckle the seatbelt, immediately locate the door handle and unlock the door. Even though it looks like certain and immediate death, your car will float for a minute or more before it goes under. You’re not panicking, are you?
Step Two: As the car begins to fill with water, lower the window; even electric ones should work for a short time after hitting the water. Escape through the opening and swim to safety. If the window doesn’t open, go directly to step three.
Step Three: Sit back and calmly wait for water to fill the car. Too panicked? Try to escape by kicking out one of the side windows. Opening a door will be next to impossible; as long as there is air inside the car, water pressure will hold the doors closed. This is known as a lose-lose.
Step Four: As the cabin fills with water, the car will begin a terrifying plunge. Continue to not panic and open the door (it will still be difficult), being careful not to break the door handle in the process. Get out and swim to the surface where oxygen is easier to metabolize for the gill-less.
I hope that helps someone one day.