Saltwater and uric acid (don't ask!) produce calcium scale inside marine toilets and discharge hoses. Such deposits cause the toilet to get progressively harder to flush and can lead to total blockage. Avoid this unpleasantry by pouring a pint of white vinegar into the bowl once a month and pumping it slowly — a single stroke every 4 or 5 minutes — through the toilet. The mildly acidic vinegar dissolves fresh scale deposits.
To keep the pump operating smoothly, follow the vinegar treatment with an ounce or two of mineral oil or a lubricant specifically formulated for marine toilets.