When you're going 35 mph and suddenly hit a 2 ft deep puddle (I've done this), that sensor isn't going to help at all.

I've used an ultrasonic sensor to detect the water level in a tank before, I don't think it would work as you describe.

Also, the sensor didn't work in that context either as condensation kept forming on it.

Is ultrasound less expensive than a moisture sensor?

The problem with both is they effectively require the vehicle to be in the water already. They need something that can tell depth before the vehicle has to slow down.