Do everything you can to get out before the car goes under. Strange things happen under water. You don't see as well, you can't breathe, the car may not stay upright. Don't assume you'll get out of the back seat once you go back there. The underwater problems I've had to do for quals are done first with eyes open so some visibility is available; then the same problem is repeated wearing opaque goggles. The attempt with eyes open is just a run-thru so you can do the real run blind. Due to turbulence, sand and junk in the water, do not expect to be able to see anything once you go under. It's not like a heated swimming pool.

How long can you hold your breath? How long can you hold your breath under(cold)water while fighting panic, smashing windows and rescuing toddlers. You have a very limited time to function before you become useless. This isn't something you will do well the first time and you won't get a second chance to do it right.

If you have an opportunity to get out before the car sinks, do it; that's as good as it gets. Call 9-1-1 after you're out of the car (but don't go back in to find the phone).