It requires adding a pump to use sea water. If you opted for the "sea fresh" system that lets you switch between sea and fresh will, the solution is simple: switch to fresh water for couple of flushes once a day, especially right before the boat will sit, to rinse the sea water out. If you didn't, it's still almost as simple: pour a quart or two of fresh water down the toilet once day, especially right before the boat will sit. That SHOULD at least slow down the buildup.

Or...flushing a cupful of white vinegar through the system once a week, should also prevent buildup...follow with a quart of fresh water after about 45 minutes...don't let vinegar sit in the joker valve 'cuz soft rubber left to soak in vinegar will cause it to swell and distort.

Letting vinegar sit the bowl is really a waste of good vinegar, 'cuz it can't help the hoses if it's not IN the hoses. If you have mineral buildup in the bowl, put some white vinegar in a spray bottle and just spray the bowl...wait about 45 minutes, then rinse.