I’ve used moth balls – crystals, actually, which we put in aluminum pie pans in a couple places under the boat cover – and they worked. But a few years back, based on advice from a fellow boater I met on a trip, I switched to scented laundry fabric softener sheets. I put a sheet in the bottom of each locker and a few more pinned down under furniture legs to keep them from blowing around. I replace them in mid-winter and the boat has remained mouse-, ‘coon-, cat- and, ‘possum-free ever since. And smells a heck of a lot better come spring.