A great article came out in this month’s copy of 48° North on techniques to alleviate this problem. Alex and Jack Wilken share their thoughts about insulation, ventilation and helpful products available on the market.
Hypervent has a non-compressible open material that allows dry air to flow underneath the mattress or you can try the Froli sleep system that not only helps with the air flow, but adds spring to your mattress.
By removing additional moisture from the air with an electric dehumidifier and using porthole drip catchers, available at SeaworthyGoods.com, to keep water off your covers, you should be warm and dry during the wet and cold months of boating. Other great ideas can be found at Pinterest and cruisers forum.