I suddenly find that I really want an answer to this as well because I'm now imagining what might ensue if one of these attempted to board a car ferry. Typically there's a sign "turn headlights off", you're expected to maintain something like 5 mph (the flow of traffic should never stop), and you get directed by a human to cross multiple lane markings often deviating from the path that the vehicle immediately in front of you took.

I think that Waymo isn't concerned about those types of scenario because they only operate in a limited area, and can tune their systems to operate best in that area (EG not worrying about car ferries, human-operated parking lots etc)