Rick's right...white vinegar
However, it's not salt...it's sea water calcium carbonates and urine crystals. The problem is worse in warm water than in cold, btw...I've seen 'em reduce a 1.5" diameter hose to less than half an inch in as little as year. Flushing with fresh water eliminates the calcium carbonates, but can't eliminate urine crystals. At least a cupful of undiluted white vinegar down the toilet once a week will prevent buildup, and it won't harm any parts in any sanitation equipment. If you already have a bad case, there are several ways to cure it. Some sailors remove the hose and beat it on the dock to knock 'em out...but if I'm gonna go through what it takes to remove a hose and put it back, I'm gonna put NEW hose back. The quickest cure is 12% solution of muriatic acid and water down the toilet when it can stand at least overnight, followed by lots of flushing with clean water. But it may take a couple of "treatments" to completely clean out the hose...and unidiluted white vinegar (white vinegar is more acidic than cider vinegar) will do just as good a job if you have a bit more patience, 'cuz it may take 3 or 4--or even more--treatments to get the job done....but it'll work. You might also want to check any other raw water hoses...the sanitation system isn't the only place that sea water calcium carbonates can build up. White vinegar is the cure for all of 'em.