In light of the horrific bridge collapse in Minneapolis, it seems like a good idea to pass along this link from Slate: How Do You Escape From a Sinking Car?
It's trickier than you would think. It's worth taking a look at that excellent article right now. And while you're thinking about it, consider buying a Life Hammer, a device for quickly breaking out a car window. That's the Amazon link from the article, but your local auto parts stores carry them, too.
You'll probably never need this stuff. But you never know.