As soon as you realize that the vehicle is in deep water and is going to sink, get your seatbelt off and try to open the door and get out. For this to succeed, you must open the door before the water level gets higher than a few inches on the outside of the door. Otherwise, the pressure of the water against the door will not allow you to open it. If you try to shove the door open, but can't, don't waste your energy on that avenue of escape.
The next best thing is to open the window and climb out before the water level reaches the glass. If you cannot open the window, break the glass and crawl out. The window glass will shatter into small fragments, so don't worry about impaling yourself. The best way to shatter the window is to use a heavy piece of metal (hammer, wrench, etc.) or a special spring-loaded glass-breaking punch. The punch takes almost no effort to break the window and you can find them available online for about $10.
If you are unable to escape before water reaches the level of the windows and begins to pour inside, you'll have to wait. Don't panic. You must allow the flow of water coming in through the windows to fill the interior enough to slow the flow and let you escape. The interior of the vehicle will need to be almost completely filled with water before you can get out. This is the most difficult time to remain calm, but it is vital that you do so. If other people are in the vehicle, do your best to calm everyone and tell them what to do.
- Get seatbelts off
- Work to get windows or doors open
- Wait until the vehicle fills with water
- Talk about who will go first, second, third, etc.
- Follow the air pocket and breathe
When the vehicle is full of water, you might be able to shove the doors open because the pressure will be equal on both sides of the door. Depending on the physical size of the occupants, that might be the only way out. If you can get the doors open while you're still breathing from the air pocket, so much the better.
When you're ready to go, take a last deep breath and hold it. Keep your eyes open so you can see your escape route. Then go.