I can't believe it never occurred to me that the area under the head sole is most likely a sump...I must have been more tired than I thought!
If the shower sump is the source of the odor, it's the easiest thing to clean....in fact, the only job on a boat that requires -0- manual labor! Raritan only markets their C.P. Cleans Potties as a bowl cleaner, but it's also the best sump and drain cleaner on the planet! C.P. is a bio-enzymatic cleaner that not only destroys odor on contact, but the enzymes "eat" hair, soap scum, body oils etc that clog up sump pumps. To use, run enough water down the shower drain to rinse out the sump, then enough water into it to cover the pump intake and put 2-3 oz of C.P. down the drain when it can stand at least overnight...'cuz the enzymes need time to work. There are no harmful chemicals in it, so it can actually remain in the sump indefinitely, so a good time to use it is last thing before leaving the boat. Just run some clean water through it when you return to rinse it out again. And btw, it's also the easiest way to keep sink drains running free and clean...close the seacock, put an ounce or two down the drain, then fill the rest of the drain with clean fresh water--again, when it can stand at least overnight. If you can find C.P. locally, the sbo.com online store has it.