Water will always seek its own level
There's no malfunction except most likely a worn out joker valve...but even a good one would only slow it down, it wouldn't prevent it. Rebuilding the toilet won't solve it either...the problem is just a basic law of physics at work: water always seeks its own level. If the bowl or any part of it is below the waterline with no loop in the line that's well above the waterline, water outside the boat will is gonna rise in the bowl to the waterline. So at the very least, you need a vented loop in the line that's at least 8-12" above the waterline at any angle of heel and new joker valve. If your toilet hasn't been rebuilt within at least 5 years, it should be...the rebuild kit includes a joker valve. The repair kit part # for PH II toilets built before 6/92 is PHRKII...for toilets built after 6/92 it's PHRKIIC. Since it's doubtful your toilet is original, I suggest you check with Raritan for the identifiers to know which kit to orider. The PH II should be rebuilt every 5 years, the joker valve should be replaced annually. If there's no loop in the discharge line, I doubt there's one in the intake either... which means that if the dry/flush valve is left open, water outside the boat will use that path to seek its own level inside the boat via the toilet bowl. So I strongly recommend you install one in the intake too. It does NOT go between the thru-hull and the pump...putting it there will prevent the pump from priming. It has to go between the pump and the bowl. I think you'll find the link below useful too.
