While staying in the boat you MUST have ventilation. Therefore heathers must be present. If the boat IS inhabitated for a period you might consider "sealing" it and use a strong deumidifier inside. - such as calcium oxide, wich is also cheap
. You might consider monthly refills. The use of highly hygroscopic compounds will make only well, if properly sealed from new humide moisture. If the case is, you will consider complete emptying of water
sistems - to avoid freezing. All inside must be opened to favorize uniform absorbtion of the moisture, but ONLY IF YOU DON'LIVE ABOARD.