Yeah, it may not have come across that some of my suggestions were a little tongue-in-cheek, and half-joking but also half-serious in that it might almost work.
This case reminds me of the similar case where a trail might sometimes be a stream instead, so similarly having the road be both a highway=residential and a natural=water (or waterway= of some sort) almost makes sense?
In practice I doubt a general router or data consumer is going to take the time to look up local tidal information and do different routing based on it, unless it’s really specialized for the specific road, even if we were to come up with and standardize the perfect tagging scheme for it. Just using hazard=flooding and/or the existing flood_prone is probably going to be about as good as one can do to try to make consumers aware to alert users that the road isn’t always passable.