Here's an old powercruiser trick:
Use a porta potty, attach a fitting and hose to the tank portion. the 1" hose goes to a macerator
pump. The porta potty is
legal as it has a holding tank. The macerator goes to a
thru hull valve that is closed in protected waters. simple easy . I seen many of them on small
power cruisers.
Another advantage is there are no internal head
parts to fail or clog. You just have to worry about the macerator. You could put a
diaphragm manual
pump in place of the macerator.