I think that it would help the game out if the games water physics was improved (example: the deeper the car goes the windows start to crack and more presser on the car to make the car to make the panels around the car dent in) I think that would improve the game in my opinion.
This already happens with solid buoyant volumes. I had to reinforce my minisub because it kept imploding when diving deeper. Cars in BeamNG are not made buoyant because the structure is not closed. I don't think what you're describing would happen to a real car that easily. The car would float until it's mostly filled with water, then the water leaking in would equalize the pressure. Impact with water would be more likely to dent the car or break windows.
