For some time, I’ve kept a window breaker / seatbelt cutter in my car in case of emergency. I don’t plan on using it, but I was a Boy Scout, and I try to be prepared.

Old Wisdom
For decades, the prevailing wisdom was that if your car is sinking, you have to wait until it’s submerged for the pressure to equalize before you can open the door to escape.
Mythbusters demonstrated this:
I can’t hold my breath that long and you probably can’t either.
Good thing there’s a better approach.
How To Escape A Sinking Car
The real key to surviving a flooding car is to get the window open and get out before the car submerges. This requires quick thinking and some preparedness.
Having the right tools on hand can be critical, especially if you have power windows. Also, don’t forget to shield your eyes.
Knowing how to swim competently can save your life in quite a few scenarios.
Avoidance is Best
Finally, if you see pooling water of unknown depth, consider not driving through it. It’s probably not worth killing yourself and your passengers or wrecking the car.
Not all escapes are recreational or fun.
A fire extinguisher is no good if it’s not handy when you need it. A window hammer/belt cutter is the same. Most of these are buried in a glove box that can’t be reached when your seat belt has tightened you against the seat. That gizmo in the second video dangling from the mirror was not only handy but it worked (at least for the window).
This is a great topic – Thank you REA. It made me search for a better way and I found some small (can go on a keychain) units for the glass and belts. I recommend others do the same. Time is of the essence so have the proper tools that are easily accessible and practice/rehearse for this scenario, especially if you have children as passengers.
This is a great point. If it isn’t handy, it might be useless.