One thing I haven't noticed (unless I missed it) in this topic is saltwater in the bilge.
As stated a dry bilge doesn't smell. But add some salt water seeping in with all its associated sea life, each dying in the bilge and you WILL get the smell of sewage.
Any of us with a salt water wash down or a saltwater going into the head can verify what that smell is.
. With a small amount of leakage via the shaft log or steering log the smell will rear its ugly head. I personally hit the bilge with some Dawn dishwashing soap and run fresh water into the bilge once a month letting the bilge pump take care of the water. This works for me.