Right, once the car is flooded, equalizing the pressure. Not against a gradient, right (as in a car with an air bubble trapped inside. But I defer to your experience...
We have a car sunk in our training pond. I can open and close the doors with ease (slowly). I know they will open with at least the air from our exhaust trapped, I've never looked to see how much air was trapped, but don't think it'd be very significant. We're practicing hooking the car up in a couple of months, maybe I'll try blowing a tank and filling the cabin roof with air and see if I can still open the door.