Nothing you flush down the toilet will cure this problem. Sea water, especially coastal sea water and some lake and river water too, is alive with animal and vegetable micro (and not-so-micro) organisms, When that water is left to sit and stagnate in the toilet intake line and pump, or worse yet, a not-so-micro-critter becomes trapped in the intake line, pump and/or the channel in the rim of the bowl, they die, decay and stink—and they stink a lot worse in hot weather than in cool weather (everything does). If you aren't sure whether that's at one of the sources of your odor problem, there are a couple of indicators:
The odor is concentrated in the head and not pervasive throughout the cabin--or at least is much worse in the head than in the rest of the boat.
The odor is much worse after the boat has been sitting for a few days than while you’re aboard and flushing the toilet regularly—in fact, it either goes away or is greatly reduced after you've flushed the toilet a few times when you first come aboard again.
You may see dark flecks from the channel in the rim of the bowl if the source is the remains of trapped animal or vegetable sea life.
To find out if this is your problem, close the intake seacock and flush the toilet as dry as possible. Disconnect the hose from the through-hull and stick that end of it in a bucket of water to which you’ve added a quart of distilled white vinegar. Pump/flush the whole bucketful through the system. Do not reconnect the hose to the through-hull yet, flush only with clean fresh water while you’re aboard this time....use cups of water from the sink.
If the odor is gone when you come back to the boat next weekend, bingo! you’ve found the source of your odor problem. If you still have odor, and it’s still confined to the head compartment, clean your shower sump.
--Peggie