If your car falls into water, you are supposed to let it fill up with water until it is quite full. Otherwise you won't be able to open the door. Once the pressure on the car door is equal on both sides, you will be able to. The trick is a) knowing this, and b) keeping calm,believing the advice is true, and having the bottle to sit in a car that is filling with water, when all your instincts are telling you to Get Out Now
That's for falling into deep water, but being in for example a flooded underpass with your car out of action, but the water isn't rising quickly. Should you stay put and wait for rescue, climb onto the roof, or make for safety by wading? Along with best practice for not getting into that situation, not playing in flood water, etc. I don't know why the new outlets don't do a little 5-minute safety briefing along with the news - it would save lives.