Life can get crazy and out of control in the blink of eye. While you should try to stay positive and hope for the best, it's foolhardy to not keep yourself informed and prepared for the worst. Knowing certain pressure points on windows the difference between tempered and un- tempered glass can save lives.
If you find yourself trapped in a car, focusing on edges of the side windows is going to be your best bet. Even if submerged in water, these these will be the best avenues for you escape plan.
The main windshield is made to crack and splinter were as side windows were made to shatter on impact if hit on the sides. When attempting to break a car window, submerged or not, the middle of the glass will be the hardest to break. Be sure to leave a blunt object in your vehicle (screw driver, hammer, wrench), these small objects can be your key to freedom.
Most people don't know that the water pressure can add up to 850 pounds of force against the window. This is where hitting it in the middle would be more than useless.
Grabbing that object and aiming for the corners are the point where you need to strike breaking out the window, filling the car with water, giving you a path to swim to safety. On land being in a pinch breaking a side window can give you also a quick escape.
Timing is everything, so keep a cool head and act quickly. The deeper your car descends the more pressure there will be, making the window harder to break. There are special tools you car buy that give you a much a higher sure chance of success. Be sure to leave it in your armrest or other easily reached area.