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.
Car ferries don't really make much sense in a Waymo-ubiquitous world. It's not your vehicle; there isn't really a reason why you would need to have the same vehicle on the other side of ferry ride. You're better off having one Waymo network on one side of the waterway, a separate Waymo network on the other side, and then a passenger-only ferry with a much higher passenger capacity (and oftentimes, they go much faster, since you can have hull forms like wave-piercing catamarans, hydrofoils, and hovercraft when you aren't carrying cars).
There are some places where a car ferry is essentially a bridge and just operate as part of the highway, e.g. there are two such instances in sacramento: https://dot.ca.gov/caltrans-near-me/district-4/d4-projects/d... The rides are about a minute long and you very much wouldn't want to change vehicles.
Another common scenario is vastly different population density on the far side of the ferry route. It seems unlikely to me that autonomous vehicle companies would want to maintain a giant seasonal fleet at such destinations.
In a lot of cases rather than seasonal it will be a surge every weekend.
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)
