I need answers to some questions before I can offer any solutions.
How old is the toilet? Does it ONLY happen in the dry mode, flushes normally in the wet mode? Is this a new problem? Or is the boat is a "new to you" boat with a toilet that isn't working right? If you've owned it a while and this is a new problem...When, if ever, was the last time (only time?) it had a rebuild kit installed? When, if ever, was the joker valve replaced?