If your boat stays inside during the winter or is subject to experience large swings in temperature and humidity, guard against an interior full of mildew by placing a product such as "DampRid" in containers throughout the boat. Be sure the interior is well-ventilated. Of course, don't store any damp gear (life jackets, anchor lines, etc.) inside, and remove all seat cushions and seats that cover storage compartments.
Consider A Solar Solution
When your boat is closed, warming daytime air sucks moisture into the interior from outside, which condenses out when the cabin cools at night. A few days of this and the interior of your closed boat is as wet as a rain forest. The result is damage to fabrics, woods, and probably even the fiberglass. Passive ventilation beats nothing, but a solar ventilator does a much better job of keeping the interior of a closed boat fresh and dry. Set to exhaust, a solar ventilator will exchange all the air in the closed cabin of a 25-foot boat every 30 minutes. Select a day/night ventilator and this circulation continues around the clock. The effect on the freshness of the cabin will astonish you.