Answer: Windows down. If you can't roll them down, break them. Ideally, every passenger can break her/his own window.
This one has always bugged me.
Traffic fatalities in 2017: 40,100
Vehicle drowning deaths, the last time anyone estimated (2004-2007): 384
Percentage of traffic deaths that are drownings: 0.96%
Percentage of vehicular drownings also involving a rollover: 63%
Percentage of vehicular drownings also involving another vehicle: 12%
So let's review: if you're in a fatality-grade accident, there's a less than 1% chance you're going to drown. If you're in that less than 1%, there's a better than 50% chance you're not wearing your seatbelt and 63% chance you've flipped the f'ing car. You are, in short, driving like a ragged asshole.
Here's the thing: for a vehicular drowning to enter your future you have to have made some pretty dire mistakes. Chances are good you're not sitting there rationally. There's been a long list of bad decisions here and remembering five steps from Popular Mechanics probably isn't going to help you out much (especially if you've rolled your car without a seatbelt on, which statistically speaking, is exactly what you just did). Yet the mindspace of this particular dilemma is huge - there are more instances on that page than, statistically speaking, vehicular drownings in four months nationwide.
That bridge in Mt. Vernon? Where the road suddenly was in the river?
Nobody drowned.