Using fresh water flush.

This made a huge difference on my boat. The boat was already plumbed to either use sea water or use fresh water from the head sink... after discussing the whole set up with Peggie, she informed me that I should just fill the head sink and use that as the flush water vice opening the valve to allow seawater in.

Works wonders. Turns out quite a bit of the smell from a head is decaying sealife in the hoses. Flush with fresh water and that problem goes away.