Comment by Zigurd

It's an interesting case of whether it's possible to infer the condition of wading and avoid having to install a sensor specific to a one in a million trips circumstance.

The inference would come from standing water slowing down the vehicle and likely require steering correction, in combination with some machine vision for identifying standing water.

Then there's the advantage of being Google and having hundreds of thousands of people in the same area using Google maps and navigation. Accelerometers in phones can detect crashes pretty reliably. There's a good chance they can reliably detect deceleration from standing water and report the location of the hazard.