Acceptable means of securing the system
In coastal waters that provide immediate access to open sea, a boat may be equipped with diverter valves—commonly called y-valves—that allow the tank to be dumped or the toilet to be flushed directly overboard at sea. The law only requires that the system be “secured from accidental illegal discharge” while inside the “3 Mile Limit;” and there are a number of ways a system can be secured; which are legal for your boat depends upon whether your boat is equipped with a Type I or II MSD (treatment device) or a Type III (holding tank). If your toilet is connected to a Type I or II MSD, acceptable methods of securing the device include --(1) Closing the seacock and removing the handle;(2) Padlocking the seacock in the closed position;(3) Using a non-releasable wire-tie to hold the seacock in the closed position; or(4) Locking the door to the space enclosing the toilets with a padlock or door handle key lock.If your toilet is connected to a holding tank, acceptable methods of securing the device include --(1) Closing each valve leading to an overboard discharge and removing the handle;(2) Padlocking each valve leading to an overboard discharge in the closed position; or(3) Using a non-releasable wire-tie to hold each valve leading to an overboard discharge in the closed position.Inland may be another story. The Great Lakes and all non-navigable inland lakes are “no discharge” waters. They’re also a loooong way from the ocean. Since there's no possible way to legally use a y-valve or macerator to dump a tank, many states have made it illegal for vessels operating on these waters even to have one or both installed. When in doubt, check with your local authorities. (The above is an excerpt from my book, btw...see link below.)