Peggie suggests just plumbing the head to the sink, that way just put water in the sink to flush the head. My assumption is the thru hull is closed to keep water in the drain line. That of course completely disconnects the water supply from the head so very little to no chance of contamination
That needs some clarification! 30 years ago several boat
builders used the same thru-hull for the head sink and toilet
intake line. I got the idea from a Tartan owner whose system was plumbed this way. It's a simple an inexpensive mod that's easy to do on most boats...the head sink and the toilet
do have to be on the same side of the keel
and the head sink drain thru-hull has to be below the waterline.
Reroute the toilet INLET line to tee into the head sink drain line as close to the seacock as possible...it needs to be below waterline, allowing you to continue flushing
with sea water most of the time, but also provides a safe way to rinse it out of the ENTIRE system--inlet line, pump
and channel the rim of the bowl--before it can sit, stagnate and stink. Fresh water poured into the bowl only rinses out the toilet discharge line...the water doesn't recirculate through the intake, pump
and channel in the rim of the bowl (you wouldn't want it to!) To rinse out the entire system, close the thru-hull, fill the sink with CLEAN fresh water (never use gray water!!), flush the toilet. Because the thru-hull is closed, the toilet pulls the water out of the sink. Or you deplete your fresh water by filling the sink with it for every flush. A totally safe way to supply fresh water to the toilet without the need for separate flush tanks
and/or pricy chemicals chemicals and gadgets that don't work
Also has the added advantage of providing a very simple way to winterize the toilet and system: close the seacock, pour the antifreeze
down the sink and flush it through. Job done.