My Watermark is the Toronto Harbour of Lake Ontario.
I don't usually like to be around the water in a large city because of this exact reason (sewage in the water). I find any water near a city pretty filthy even if that water is part of an ocean.
For example, New York, Boston or Miami or Halifax Harbours are not any cleaner. Harbours in any city is usually filthy as all the sewage tends to end up there, but being a lake harbour, I find the Toronto harbour extra dirty because of the lack of cleaning systems; however, it would be easier to control and monitor compared to an ocean harbour.
In Bangladesh, a developing country, there are small private voluntary organizations that focus on the Coastal Cleanup, meaning volunteers picking up garbage from the beaches and making it cleaner. But here [in Toronto], the government can get involved and make it a faster process.