I put a "T" in the sink drain. We flush by running water into the sink and pumping it through. In addition to reducing odor greatly, the method also allows good control of water used in flushing - i.e., how much you put in the holding tank. After a little trial and error you can figure out how much water to put in the sink for various flushing needs. If you put too little in, all you have to do is run a bit more into the sink.

In your case where the boat will be tied to the dock, I would use fresh water flush exclusively. Your heads will be much more pleasant that way and you can manage flush volume to reduce pumpout frequency