Neighbor of mine had all his seats destroyed by raccoons one spring/summer.
He had a tarp over it, but not shrink wrap, so they easily climbed in and out.
Left a mess of #2 and smell of #1 as well.
My brother and I have been storing our boats in our parents garage for 30 years. No moth balls, dryer sheets or any other repellants used.
Nowhere for the mice to get in, unless they slip in when the door is open. Never had any indication we had mice in there....ever !
A barn is different.
I've stored boats with no cushion seats or upholstery outside. Tipped up for the rain/snow to run off and always good to go.