If your boat’s heads flush with salt water, the plumbing can (and will) accumulate salt deposits in the discharge channels and hoses. These deposits will cause your head to get harder to flush over time. Salty scale on the valves might also allow waste water to leak back into the bowl. Left unchecked, salt deposits will eventually lead to total blockage of the head system and a very unpleasant experience.
The good news is that there’s a simple solution to the problem. Just run white vinegar through the head once a month. Pour about a pint of vinegar into the bowl, and then give the head a single pump every 4 or 5 minutes to slowly move the vinegar through the system. The acidic vinegar dissolves fresh scale inside the head and hoses. When the vinegar has passed all the way through the system, pump a gallon of fresh water through to flush the lines.
A clean head makes for a happy boat.