On the surface it would seem to be relatively easy to establish a baseline for unpolluted water and then prohibit introduction of "things" in quantities that cause significant and long term changes to the chemistry of the water that impact the biochemistry of the water system. The problem comes in with that word "significant". That is a tough one to define particularly if economic factors are considered, which they often unfortunately are.

Of course the bigger problem is that the vast majority of people don't have an iota of common sense and would happily discharge raw sewage directly into the waters in every marina if it wasn't prohibited simply because of convenience