Phew !!
Betsy,You should read the informative articles that Peggy Hall writes in the Head Mistress Forum, that will answer a lot of your questions and make you more knowledgeable about marine sanitation. I think your boat is in salt or brackish water. I feel sure the smell you are experiencing is from the intake seawater marine organisms. Every boat I have had I experienced the same thing when first pumping the head after not using it for a week or so. When I arrive on the weekend, and flush the first time I experience the foul odor. It is embarrassing when your guests are the first ones to use the head! The smell comes from marine organism little sea animals that are in the intake water line of the head. Once you flush a couple of times and air out the head area and use it regularly you should not experience any smell.I had a clear intake line on my previous boat and the little sea creatures would glow green and phosphoresce when pumping the head in the dark! The other smell you could experience might be the holding tank vent if you smell it out in the cockpit when flushing the head. I personally strongly recommend the head cleaning and maintenance chemicals that you can buy through the HOW emporium from Peggy's company. I use C.P. (Cleans Potties) You will love the clean fragrance, and K.O. (Kills Odors) K.O. is a bacteria that you put down the head and it goes to work in the holding tank if the tank is properly vented and eliminates smell from the contents. Follow the instructions for use! Good luck !!