...is something we should all have an idea how to accomplish, especially in light of the recent tragedy in the Minneapolis, Minnesota with the bridge collapse. I have reviewed a lot of the news about this sad event, and I was very surprised I did not find one article suggesting how to escape a submerged vehicle.
I have some knowledge of how to do so based upon my employment training, watching documentaries, and watching Myth Busters (a recent show I saw) where they explained the how to get out part, and also why you sometimes cannot get out until the car is full of water.
The following is my understanding of what needs to be done, should such ever happen to you. I am not sure it is 100% correct, but it will give you an idea of what to do; but remember it is your own responsibility to check with proper authorities to verify these steps.
If you are ever in a vehicle that winds up in the water completely submerged, there are a few steps you can attempt that may greatly increase your, and other passengers, chances of survival. The first thing to do is to remove your seat belt (always wear your seat belt when driving, it may prevent you from being knocked unconscious if you ever wind up in a submerged vehicle, and then what could you do to save yourself). Then tell others to do likewise, or remove the safety restraints of any other young occupants who cannot do so themselves. Then tell others the plan. Then take a deep breath and hold it, then open a window (or break it open, try to break a side window and not the windshield as it is much stronger than a side window), if not already open, and allow the water to flow into the car, and do this as soon as possible. Then either escape through the open window or open the door and get out; and of course if need be, you first try to assist any young children, or handicapped people, in the car to get out. This goes even if you cannot swim, because you can float to the surface. If you stay in the car, and help does not arrive almost immediately, you probably will either suffocate (if the car does not fill with water, and it not eventually filling with water is highly unlikely, or you will drown once it does fill with water).
The reason you wait for the car to pretty much fill with water is that you probably cannot get out until it does because of the water pressure. Until the water pressure is just about equal outside and inside the car you most likely cannot open the car door, nor could you get out of a window with the water rushing in. This is why you take as big a breath as you can, then open the window, holding your breath as the water rushes in. Once you can get out, you make for the surface.
By the way, if your car winds up in the water, there is a chance it will float for several seconds up to minutes. If so, get out through a window. Do not wait for the car to submerge, get out.
A good thing to carry in your car is a 'center punch'. It is a pointed, spring loaded, pen like device used to mark areas where someone wants to drill or hammer nails. If you have it set properly (usually as set when you buy it) and you press it into the side window of a car, the window will shatter. This device should be handy, as in a small case that is securely attached to your dashboard, or center console, for easy access.
Will this plan always work - probably not. Is it better than just staying in the car and doing nothing - probably so. As for me I would bet my life on it.
All the best,