There are local and public boat services (ferries) around the world, similar to:

http://schema.org/BusStation
http://schema.org/BusStop
http://schema.org/TrainStation
http://schema.org/BusTrip
http://schema.org/TrainTrip
http://schema.org/TaxiStand

What is missing is a possibility to mark these up, and the time tables.