Is it? I have been driving for 25 years and never encountered one.
Waymo seems to accept they can’t predict everything so they built a system that’s safe enough to operate in the real world and learn from experience.
I haven't encountered one as a driver either, but I'm pretty sure "Don't drive into roads with water on them" was a basic safety question on the permit test.
You haven't been driving for 25 years anywhere east of the Mississippi river if you've never encountered road flooding. Accepting they can't predict everything sounds reasonable. Failing to account for a routine occurrence is negligent.
You probably haven't been driving in areas that flood then.