Tee your head intake hose into your head sink drain line. While you're on the boat, flush with seawater...when you leave the boat, after you've closed all the seacocks, fill the sink with clean fresh water...flush the toilet. Because the seacock is closed, the toilet will pull the water out of the sink, rinsing all the sea water out of the whole system--intake line, pump, channel in the rim of the bowl AND the head discharge line. You will need to keep a plug in the sink except when it's actually in use...otherwise, the toilet will pull in air, preventing it from priming. However keeping a plug in the sink shouldn't create much of a hardship since very few people use the sink and the toilet at the same time.

Btw "OdorSafe" brand hose only comes in one size: 1.5"...head intakes are all 3/4"...so how'd you manage to use OdorSafe in your head intake??