if not treated.

I have been sailing, and also working in industrial prolluitn control for over 40 years. The differnece in many rivers, lakes and bays in that time is dramatic, since most municipal and virtually all industrial effluents are now treated quite well, or very well.

Of course one boat dischargin in a reasonably open anchorage is no problem. However, several can be. Marinas and tight anchorages with little tide are of course the most vulnerable.

It is not as simple as human crap and its immediate impact. Nutrients build up over a few months in any water not well flushed, cause algae to grow and the water clarity to go to hell.

I find it hsard to imagine many anchorages turning brown, but have not been in the Caribbean for some years