By a water sensor do you mean a sensor to detect the water level relative to the chassis? It seems like a very inexpensive downward-facing ultrasound sensor could work.

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.

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.