Yes, I understand. We also have a sailboat on a lake. It has a portapotti and gets dumped in a sanitary manner, not in the lake. It also has chemicals added to it for usage while we are out on the boat.
I was referring to raw sewage at a resort on land being used for irrigation and then tracked into a pool. While the pool was highly chlorinated and I personally never had a problem, there were always several kids ill with GI symptoms every week when we were there.
Grey water from washing is another thing altogether.
Sewage is not harmless. When a nation like Tahiti, which is not exactly 1st World, bans all swimming in its capitol and admits to polluted waters, then it is not exactly a myth. We spent our vacation time on Moorea and the water was fine, as far as I know. The ocean is quite shallow and warm off Moorea, at least where we were. OTOH, we were also there another time, way back in the 1970s, when none of the island residents had plumbing. Every morning, bright and early, they would traipse up to a high point on the windward side and use a primitive outtie that was open directly to the sea. Lots of wave action there and if there were problems, its a certainty no tourist was purposefully told about it.
As for legal: is it really? Our local water cops would fine anyone they see dumping feces into the lake water. I am pretty sure it is a real law on the books. Of course, out on the open sea, no one knows.
I said gray water. Gray water is not sewage. It is never legal to dump sewage under the mileage limit.