Water on maps isn't the reason its when the car goes below y-0 does it happen and i think it's caused by there being a body of water at that coordinate that is generated by default and is removed by a map creator but is still saved by the game thinking if you go below y-0 your car "drowns" but it isn't caused by the engine air intake it's the center of the vehicle that is affected by it because I used a car with a snorkel and it still drowned at the exact spot other cars were at.
Punctuation. It's a know bug, and by the looks of it no one really knows what causes it. I haven't experienced it. Probably because I only play on the grid testmap. :|