Freshwater systems can be drained and if a gas geyser is fitted, remember to drain it also. Gas supply should be isolated and, finally, give everything a really good clean.
A range of new and portable dehumidifiers have come on to the market over the last couple of years and these can be great gadgets to keep boats free of damp but such devices come with a warning. Several fires have been reported involving dehumidifiers. In one instance in Howth, the fire was spotted in time and the main damage apart from the unit itself was smoke damage and fire extinguisher residue internally in the cabin. If you intend to use a dehumidifier check it and associated electrical equipment to ensure their serviceability when left unattended for long periods of time.