If your headlights appear dim
Apply toothpaste to the light covers using a lint-free paper towel, then rinse with a sponge and warm water. The mild abrasives in the toothpaste will remove it without damaging the road surface.
If you’re trapped in your car
Note that most vehicle headrests are removable. If you can’t open a door or window, you can try to break a side window using a metal bracket on the headrest, but this may not be possible in a submerged vehicle. Tempered glass is intentionally made difficult to break, so good luck!