We live aboard in Portland and we do a few things to tackle the humidity. First, our heater is a diesel fired hydronic Hurricane system which does not add any moisture into the air. We also run a dehumidifier once we stop sailing for the year after Thanksgiving. Hypervent works great under the mattress. Cut out blocks of foam to fit into the hatch openings.
In the winter, we cover the boat with clear shrink wrap. We leave a small opening at the toe rail for ventilation. It can get into the 70s and 80s on a sunny day. We take this opportunity to open the hatches and let in some of that dry Maine air.