I don't think you can just rule out liveaboards, weekenders are pumping overboard too.
IN many parts of VA, those pumpouts just move it from spot a (the boat) to spot b (the marina septic system, or worse the sewage line).
Spot B is where the major stuff is coming from. MD and VA each pumps tens of thousands of gallons of stuff directly in to the bay, each and every day and that is the "normal" operation....when something fails....that number jumps to hundreds of thousands of gallons.
I ran some numbers way back, about how IF every boat that is registered in DC/MD/VA, regardless of length, location, use, etc - was assumed to have a 15 gallon holding tank, said tank was full, and said boat was sitting on the bay AND they all dumped their load at the same time, it would not be any worse than the sewage treatment plants going in to overload and "spilling" the overload during one sunday football game, which Baltimore and DC do on a regular basis. As well as some of the landlocked cities further upstream, that have been known to have major infrastructure issues, if they have any infrastructure left at all...
While the gross factor is there, think about the bottom feeder critters who thrive on that stuff. If you understand what crab, catfish and shrimp actually eat....
Well you get the picture. Politicians want the common people to blame those "rich boaters" for the pollution, as it draws attention from the real cause.
runoff from those mega malls and parking lots
runoff from AG activity (perdue, etc)
sewage treatment activity, deliberate and leaks