Here's a few ideas. Look for a product called cab fresh at local farm supply or online. It is mesh bags filled with natural repellents to keep mice away. It has a strong odor but doesn't smell bad. I use these in combine and other equipment I store on farm for the winter as well as my boat. Another is buy some peppermint oil (grocery store near the spices) and pour some onto cotton balls and place in your boat. I always cover my boat in the winter with a sheet of clear plastic instead of the cover and also prop all my storage doors open just a bit so I don't create a dark hiding spot for mice during the winter. I remove life jackets and and anything I think a mouse could chew on or make a nest in and store that stuff in plastic totes.I have a couple buddies who do the same and even remove their engine covers so mice can't get under them and have a heyday with engine wiring. Always seemed to me the more light and exposure something has the less mice like to bother it.I use poison as well but I just put it in plastic bait station boxes around my shed and one box right underneath the boat.