If you have a large enough boat, the boat will have a marine toilet or porta potty. The marine toilet pump or marine sewage pump will pump from the toilet to the holding tank. You will need a boat pump out system for the holding tank. In the great lakes and tributaries most marina’s offer free or low-cost pump outs.
Along our three coasts marine pump out facilities are harder to find, as you can go 3 miles off shore to dump. Your holding tank must have a macerator pump or vacuum pump to get the effluent out of the holding tank and into the sea. The marine sanitation pumps work great if nothing is added to the holding tank other than what was meant to be in it.