Once a car is in water over the wheels there is nothing to worry about but getting to safety. The engine will stall and the car will some to a drifting stop. Bail out the window and look for the safest place to jump and or swim. A late model car is a total loss once water is up to the level of the drivers pedals.
I have had a few hours of fun driving jeeps equipped to operate in water over the hood and as soon as water was spilling into the floor board there was no driving to be done it was just a matter of keeping the vehicle upright and helping it toward shore. When you have nothing to lose it's a lot of fun. When you're driving an investment that is the family's daily driver it's probably no fun at all.