AWOL:
My car has both front and rear ultrasonic sensors.
I would have thought that they'd be at least IP65 to withstand rain hitting them at over 160 kph.
The issue is not if a sensor that can withstand rain (obviously, they can as weatherproof ultrasonic sensors exist). The issue is will it actually work while submerged. Put your car under water, back up, and see if the sensors works. I would guess they won't. The ability for the sensor to withstand rain at 160kph is meaningless in this context. Will it work underwater is the question, and your car sensors I'm sure wouldn't. But, I would suggest trying, just to be sure 
Tim