my boat stinks
The small amount of seawater that remains in the bottom of the bowl is often the culprit. While it sits there from Monday to Friday with summertime temperatures, the tiny creatures present die and decompose -- with a predictable result.
Try this and see what happens. When you''re ready to leave the boat and have closed the seacocks, soak up that last bit of water with paper towels and leave the bowl perfectly dry. When you return, your nose will thank you.