It's not just the winter. During the summer months it sits in the slip with the cover on it, and it builds enough moisture under that cover that it will start developing mildew spots. Those are easy, but the ones that develop during the SC winter are much worse.
I'll probably pull it out of the water this year for those four months where we don't use it much. Maybe that'll cut down on the moisture and mildew.
I could just leave it uncovered and let the sun and rain take it's toll. Probably no mildew, just would have that fresh "sun-baked" appearance after a while.
What kind of boat is it? If you've already told us, I apologize for asking...but my
memory sucks.
It's a 25' tri-toon. The cover does have vents, but they're just four small ones where the tent posts plug in. Talking about it with a boating friend, we think it's worse because the floor is carpet covered wood, there's a large area under the cover that doesn't get as hot as a fiberglass boat so it doesn't dry out as much, and it's sitting out on the water year-round so it's exposed to moisture constantly.