Most boaters pull and winterize their boats before the onset of cold weather. If your boat has been shrink-wrapped or has closed-off areas with little or no ventilation, the trapped air contains much more moisture than the cold winter air will hold.
If you’ve taken the USPS Weather course, you understand relative humidity. For example, air at a temperature of 70 degrees Fahrenheit will hold more water vapor than the same volume of air at 36 degrees Fahrenheit. This means that when you seal up your boat at 70 degrees in the fall and the temperature drops to 36 degrees in January, the excess water vapor condenses as it does on the outside of a glass of iced tea.