I suspect that if people continue trying to improve self-driving cars, they will become closer and closer to building a boat (if building such a boat were necessary under the circumstances, which seems unlikely). For instance, it wouldn't be that far from what we have for the car to check whether there is a ferry and go to it. That might be improved over time into finding a boat-supplier and buying a boat if there is no ferry. If the boat supplier were also automated, and they were in close communication with each other, it isn't that different from your car being able to make a boat.