The Cure...and the Prevention
The buildup isn't solid waste (at least very little of it is)...it's sea water calcium carbonates that are stained by waste. The cure: several applications of undiluted white vinegar down the toilet...or, if you're in a hurry, a 12% solution of muriatic acid and water. If you go that route, be sure to follow all directions for use VERY carefully! To use either one, disconnect the hose from the tank and plug it first, to keep the solution in the hose. White vinegar should stand several hours...the muriatic acid solution only about an hour. Prevention: Once a week, or last thing before the boat is to sit between weekends, flush a couple of quarts of clean fresh water down the toilet, followed by a cupful of undiluted white vinegar. Be sure the flush the vinegar all the way through to the tank...do NOT leave it sitting in the bowl.It's also important to make sure that you flush the toilet long enough to move bowl contents ALL the way to the tank. Don't stop pumping or take your finger off the flush button as soon as the bowl is empty...that leaves waste sitting in the hose to permeate it. For more details about how to cure and prevent most sanitation system problems, check out the link below.