Holding tank has nothing to do with it
Most likely, the head intake is the source of your odor. If only the first flushes after you've been away from the boat stink, it's caused by stagnant sea water trapped in the head intake. If it doesn't go away after you've flushed a few times, you may have pulled some animal or vegetable sea life into the intake that's gotten trapped in the intake hose or channel in the rim of the bowl and is decaying there. To clean it out, close the intake seacock...disconnect the head intake hose from it, and stick it in a bucket of clean fresh water...flush the whole bucketful through the toilet. You can reconnect the hose, but leave the seacock closed...flush by pouring fresh water from the sink down the toilet for the whole weekend. If we've cured the problem, the first flush using water from the sink when you come back aboard next weekend shouldn't stink either...if it does, we need to find and remove whatever has gotten trapped in the intake hose or channel in the rim of the bowl.
