The problem was you had nice beaches and gathering spots on shores along federally regulated waters (oceanfront, large rivers, certain lakes) and folks in motorboats would pull up just offshore and dump their nightsoil buckets into the local swimming hole right in front of everyone.
The law Congress passed would allow local communities concerned with the condition of their beaches (if not the propriety of folks just dropping trou to dump in front of an audience) to petition EPA to set up a zone of control where such emissions would be prohibited except by permit.
Next thing you know you had EPA telling Coast Guard to direct all small boats to cease carrying port-a-potties and to install permanent type toilets. These orders were issued under penalty of pain of law, confiscation of boats, imprisonment, fine, and maybe even keelhauling!
With the expansion of the national boundary from 3 miles to 200 miles, even large oceangoing vehicles suddenly found themselves in the range of these regulations ~ and EPA elected to attack scully work and began regulating what happens to cooking oil in pots used for frying fish!
It only got worse. By the time BP came along EPA was directing Coast Guard to prohibit oil skimmers from coming in to remove oil from the ocean ~ the problem being that the process involved also putting a small part of that oil right back into the ocean.
So, whatever EPA agrees to when it comes to the Clean Water Act you have to look closer and see what they're doing with the "dropping trou" zones! That's the basis of their regulatory authority over oil.