Years ago, when all boats pumped directly overboard, you had to look carefully before diving in for a swim. Great Salt Pond at Block was a perfect example...plenty of trapped, floating turds. Now the pump boat comes around frequently and the harbor is clean. This is a good thing.
I think this is the most important counter-argument to those who claim that boat discharges are negligible. Yes, once fully diluted, the discharge may be negligible. But at the time you discharge it, it is highly concentrated, and it can take time to mix with the other water. In a closed cove, it could hang around for days. But even in an open body like the Chesapeake, it could take hours, during which time someone could swim or waterski through it. So it might be negligible compared to other discharges, but not to the person who comes along at the wrong time.
...I sailed all over the British Virgin Islands where the water is NOT nasty and no one would ever think to use a holding tank there either.
Actually the water can be pretty nasty in Roadtown. Go into the Moorings/Sunsail/Footloose marina and walk back into the back part where they keep the Footloose boats. There's a vile smell there. I don't know whether it's from charter boats leaving their T-valves open, or from Roadtown discharging sewage into that area, but it's a most unwelcome odor, and I suspect that's why the most upscale Moorings yachts are kept at the outer part of the marina.
I've also read that things get pretty stinky over by Conch Charters, but I have not been over there to personally witness it.
My point is that you can even stink up paradise, especially in populated areas.