Flaw in the logic.
The waste that is pumped out to be treated in the municipal sewer plant reenters the sound. When it does, according to figures from the EPA, Seattle Metro, etc, it is measurably dirtier after treatment in the municipal system than if it is treated onboard.
The stuff we flush from home, or pump out into the marina pump out, doesn't just magically "disappear", it winds up right back in the sound. Less effectively treated.
So it's all going into the sound anyway, the only difference is whether it should be legal to treat it on board, to a standard *higher* than municipal treatment achieves.
Nobody, at least that I know, is advocating that raw sewage should be a permissible discharge.