this might have already been discussed , what about inland waterway vessels? such as narrow boats or inland pleasure craft? i am a comercial skipper and command a 100ft inland passenger vessel on the upper thames. If you think driving a container ship into rotterdam is hard , try navigating a smaller vessel on the thames with 30 row boats and rowers and pleasure craft around who do not know either sound signals or the rules of navigation on the river, all that and your also in a non tidal waters but do have varying currents and wind to deal with.