I agree and that is where I got the idea to use Odorlos. I've been using it for many years.
I will add this - If you are flushing with raw water, you are sucking marine plants and animal into your head with the flush water. They dry out and die in the rim of the head and the next flush will bring them and their odor into the head (and boat). If the odor is inside the boat when you flush after not using the head for a few days, this is probably the reason. Installing a filter screen between the thru hull and the the head will help somewhat but the real cure is a fresh water flush head.