That's odd Matt as I don't know of a C-dory that does NOT use a raw water flush! Our 25 comes in the sea cock and either goes to the wash down or the head.
On to this stink thing, we boat exclusively in the salt, use the head for a lot more than urine and are very happy with the system. We learned early on how to stay happy with the system.
The only additive we like is the West Marine Mint version, the others contribute to their own funky smell.
As a trip comes to a close we will dump the tank (yep - in the ocean) and travel a ways with the sea cock handle open and the toilet valve in the wet bowl position. This will fill your holding tank with little or no effort and with a little practice you will figure it out. Pay attention to how full the bowl is and shut the sea cock if it gets close to the rim, only takes a second and the water is gone and you can open er up again. It's more a function of speed and we find that at 18 knts everything works fine.
Fill that holding tank and slosh er a bit before dumping it again (in the ocean). We do this twice before taking the boat out and once home I do the same thing twice with fresh water. The last time I leave the tank full and add 2 oz of the mint stuff and just leave the thing sit full till the next trip. Lack of oxygen prevents funny growth!. Ive been using this method for the last 3 years and I would chalange anyone to smell my head!
It's a lot simpler than most folks make it.
Mike