So the first waymo to get to this less used road to john’s will not have the data rather than every waymo that travels down a new highway, that then becomes a problem if it rains.
One car with an issue of first coincides with rain on a less used road?
Well, it's closer to: any car with stale data and sufficient water depth is a financial and PR disaster. These cars are not cheap, and a tiktok of someone being driven into the water is even more expensive!
Sure, if you drive around slow enough so you can stop in time. Lets say coefficient of friction is around 0.5. That means you can drive around town at a brisk 12mph, if you need to stop within 10 feet (with 0ms reaction time).
People usually drive slow too through flooded sections. They don't have the advantage of any descent of the car diverging from HD maps, or any water level visually below HD map surface serving as an immediate warning.