It is an available service, it does not matter if its 'official' or not. There is even a sectin within the amenity=taxi wiki-page - https://wiki.openstreetmap.org/wiki/Tag:amenity%3Dtaxi#Taxi_Boats 

Also, i think its perfectly fine to map it with ferry after reading https://wiki.openstreetmap.org/wiki/Ferries - although i think taxi_boat might be a better fit, with a bicycle=yes 