I think there are two issues here. The original post said that everyone on deck knows you have used the head. This happens because gas is forced out of the above deck tank vent as the liquid enters the tank. You can install a filter in the vent line to capture the odor. Search the Internet for a Sealand SaniGard Holding Tank Vent Filter.
The rotten egg smell coming from the head itself during the first few flushes is likely from the intake seawater. When you don't use the head for a while the microorganisms in the seawater breakdown anaerobically in the intake hose which creates the sulfur smell. When you flush you pull the stinky water in the bowl. There are few solutions. Add a Y valve (and a check valve) so you can pull water from you tanks for flushing. You only need to use the fresh water for the last flush or two before you leave the boat. Add the valve as close to the seawater intake as you can. Or you can by a small inline treatment item that adds a chemical to the intake water as water flows past it. Or you can open a hatch when you arrive at the boat and flush the head a few times. As long as you use the head twice a day or there shouldn't be time for the anaerobic decay to to happen.
Right on target response from you again, Steve.
However, I will warn you that once you get a colony building up in the intake (NOTE
: INTAKE - NOT WASTE) line that it takes almost forever to get rid of it.
I have the exact same head you have (in fact, two) and just rebuilt last week. First of all, you can stop the wter coming around the pump handle by putting a 1/4 turn on the plastic nut. Pump it. If still leaking, do again. It has a rubber washer in there with packing materieal on top of it. The nut is made to be adjusted over time to address the wear of the packing material. That should stop your leaking around the handle.
Second, to get rid of the intake line smell, you have to constantly use the head. We did not find, even as liveaboards and using all day, that it totally went away once a good colony was in there. We tried inline deoderizers, but they were expensive and their success marginal. By using it a lot, you are keeping a large buildup/decay from growing in there - but it still will, so that will not solve the problem either. What I have done is very different. I bought a groco strainer and break up a HOUSEHOLD Cl tank drop-in treatment (Made by Clorox, but there are others) into the strainer inline. I also drop a Teflon Blue Tank Treatment in there to help coat the rubber gaskets. THis should not hurt your hoses, but will cause some premature deterioration on the rubber and plastic gaskets/pieces inside the head. Mine has been in for about two years I guess. We used it for about a year before that. So, plan on a rebuild every 2-3 years if you use this system. However, there is NO (read NO) smell in our head and it actually is VERY pleasant.
Third, you mentioned an enzyme treatment. THose require your tank to be Cl free and chemical free or you deteriorate the enzymes or bacteria and they become ineffetive. If you are going to set up the Cl system I talked about, you will not be able to use those. But, those are really expensive and only work marginally (especially in colder climates). Best bet is a basic chemical (and environmentally safe) treatment. It is also the cheapest. It will help break down the paper and waste in your tank, along with some deoderizing to help mask the smell. The CL will also helo, should you go that route, to keep any bacteria growth down to a minimum.
After that, just constant pumpouts (with several flushes of clean water) will take care of your problems... at least to the point that it is tolerable for you and those around you. You will also finde your boat smells a LOT better down below.
Hope that helps,