I drive a volvo wagon ... my exhaust pipes go under water all the time. I don't leave the car running while I unload, though - turn the engine off and set parking brake. I could put my back wheels under water and I know that I'd have no fear of water getting far enough up to the engine ... however, I don't know what kind of back pressure that would create for the exhaust, or where the exhaust would go if it couldn't go out the pipe ...