The intake side of your toilet is probably contaminated with rotting vegitation. Seaweed can be picked up with the flush water
, it gets caught on the intake side of the toilet and rots. I usually find it caught in the rim of the toilet bowl. There is a large passage
in the rim for the flush water
. There is a series of small holes under the rim that lets the flush water into the bowl. The holes are too small to let the large pieces of seaweed to come through.
Everybody looks for the source of toilet smells on the downstream side. If you smell it in the bowl and your joker valve is good, the smell is coming in with the flush water.
The fix is to clean the intake side from seacock to the bowl rim and install a strainer to catch the larger bio stuff before it gets caught in the rim of the bowl.
You should read Peggie Hall's book "Get Rid of Boat Odors" for a more complete explanation.
You can order it here: Get Rid of Boat Odors by Peggie Hall