Im afraid one doesnt like to dwell too long on the topic of sewage,
but the daily ferries must need emptying too but one prefers not to think about all that too deeply. Presumably there is a method for extracting same as I would imagine its not all dumped into the sea...I think it was Diana who mentioned previously that those smaller ships floating around the harbour with the long nozzles sticking out do the emptying but I dont know for sure.
But one hopes it isnt all emptied at sea.