I strongly recommend taking your seatbelt off asap if you're heading into deep water. Those seconds you spend fumbling with your belt can mean seconds under water -- with mounting water pressure. While I don't have first-hand experience, I once knew a lady who did end up inside a car, sinking into a deep lake, after having hit ice on the road. She said not having her seatbelt on saved her.
Good point Mark. Just wait until the initial impact with the water is over, no good being knocked out.
We used a HMMWV trainer about three years ago that flipped you upside down in the simulated drivers seat of a Humvee. Pitch black water, belted in, body armor, etc. It is disorienting to say the least. Even though you know it is coming, it makes your heartbeat a bit faster. A couple guys had to be assisted by the safety diver.
Also the seatbelt cutter/glass breaker displayed earlier works pretty good. Start by punching the glass about two inches from one of the corners and move towards the center about two inches. Usually by the 3rd time it shatters. Under water pressure it may shatter immediately, but I've seen it take twice too. Don't use it on a windshield, it doesn't work on laminated glass.
The seatbelt cutter is a necessary tool too. I've worked major accidents where the seatbelt did its job but was jammed from the impact and would not release the regular way. The belt cutter slices right trough it.
Attach the tool with 550 cord and a quick release know to your visor. That way it is within reach when you need it. Fishing around in your console or glovebox could be disastrous.
