Weeell, if you were around here in Massachusetts
, the local harbor master provides that service!!
Yes sir he does! If you make a reservation, he can pump your boat on any Saturday or Sunday morning (only) providing it is not a holiday weekend. But the town and an adjacent city pumps as needed into the harbor. OH, they call it treated waste, but you cannot treat your own and dump it.
Or you can just sail/power off to the end of the 3 mile limit which begins 15 miles from shore where our off shore islands are.
There is one other way but I prefer not to go into it.