Where does someone's dog do their thing? Where does the cats' "stuff" go? All the wild animals and birds do their thing outdoors, I believe. And just exactly where do you think all that "s@!^" goes when it rains?
A half dozen liveaboards are not the problem.
When the sewage treatment plants have problems they dump massive amounts of untreated human excrement into the rivers and bays of the US, secretly and without fanfare.
The aged sewer system in Ft. Lauderdale leaks huge quantities of raw sewage into the waters of the Las Olas Isles area, but the city blames the yachts for all the pollution. In the late 60's California was all over the liveaboards in Sausalito about pollution while the farms and factories up the rivers were dumping huge quantities of truly toxic stuff in the rivers, which then flowed into the bay. I'm sure it is the same all over the country.
Most liveaboards are extremely conscious the environment we live in. We were using biodegradable products long before mainstream America had ever heard the word biodegradable!.
I get so tired of being the target of every government agency that puts pollution on the boaters because their administration doesn't want to foot the bill to repair or replace aging sewage systems.
Whatever we may or may not be putting in the water around us is infinitesimal compared to that dumped by modern society.