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.
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.
Unfortunately, those get screwed up in climates where there is salt used on the roads.
Also, the sensor didn't work in that context either as condensation kept forming on it.