If the LIDAR can sense the road close enough to the front of the car, then it could estimate how far underwater the car is.

There are better ways to measure how far underwater the car is: https://www.landrover.com/ownership/off-road/wading-depths.h..., this assumes you driving over something more or less flat underwater and that car is built for this.

Here the goal is avoiding driving into the water in the first place.