"3. Keep the Wave at Your Back"
You are now in escape mode. Follow the generally-accepted water escape rules. If you're truly prepared, you have an emergency hammer or window punch and seatbelt-cutter combo within arm's reach to break the window. If you can't kick it open (the Mythbusters couldn't), you'll have no choice but to wait until the car is entirely submerged, so the water pressure inside is the same as the outside. DON'T PANIC, or you'll use all your oxygen — and you'll need it.
I always feel like this style of post coming so soon after a major disaster is in questionable taste. They're a kind of survivalist corollary to ubiquitous disaster porn, part of the winking fake-stern lineage of The Worst Case Scenario Handbook, that seems to say "wouldn't it be exciting to need this information?"
In a certain sense you can't deny the utility--this is useful information, after all, and it can save lives--and yet I can't escape the feeling that there's a kind of voyeurism of the imagination in play.