I suspect your toilet is just worn out
And there could be other reasons too. On a 22' foot, the only W-C model the head can be is a Headmate, which is a low-end toilet that only has an expected lifespan of about 7 years...maybe 10 if it's rebuilt every 2 years--which is highly unlikely. If yours is original, it's 18 years old...the pump cylinder has to be so worn, scratched, and scored by now that new seals won't seal it any more. However, if it's between 3-5 years old, there could be another problem: it was defective to start with (if you bought it new, and it never worked right, WHY didn't you just return it while it was still under warranty???). W-C's previous ownership was having their toilets built in Costa Rica...hadn't retooled in decades...and a lot of the parts no longer met specs, which meant a lot of defective toilets. Either way, I THINK you need a new toilet. But maybe not...there's something else you need to check out first. The hoses relative to the waterline on the boat have nothing to do with whether a toilet flushes efficiently or not, only with whether or not your bowl or tank can overflow if you leave a seacock open. But the length of the hose to the tank might. And so might the condition of the head discharge hose, especially if you're in salt water...'cuz if you're like most people, you stop flushing as soon as the bowl is empty, leaving waste in the hose wherever it happens to be when you quit. That can lead to partial blockages, especially if the tank is more than a few feet from the head. It can also result in urine crystals and sea water calcium carbonate build up in the hose (which also traps waste)...I've seen that reduce a 1.5" hose to half an inch in a couple of years. When was the last time you checked the inside of the hose between the head and the tank? Unless the hoses are relatively new, your problem may be a hose with a diameter reduced so small that it's preventing anything from going through it at the rate the pump is trying to move it...so it just backs up. Check out the hose...if that's not the problem, replace the toilet.
