From what I understand the apparent inaction by the regulators in the US forced a lawsuit which the government lost, and the US EPA was pushed on its heels to enforce its laws, and to come up with some action. Their action was the VGP, which like DWinsor states is quite a piece of work. There is quite a bit of activities especially with paperwork around the VGP, including considerable paperwork, weekly inspections, and logging, crew training, etc.
Essentially - this may have changed - but like DWinsor says there is no discharge overboard. Usually the grey water is the problem, as black water treated by an approved MSD (Marine Sanitation Device) is okay, as I understand it.
In practicality, for the length of time we were operating in US waters, we would hold our waste, until we tip toed back into Canadian waters, as I was not sure which state you could or could not, and what to discharge.
here is the link to the US EPA's VGP website. https://www.epa.gov/vessels-marinas-and ... d-laws-and