If your home and your boat are one and the same—and the cruising kitty doesn't have enough left to get you south to warmer climes this year—then spending winter aboard looms as the likely scenario. Apart from the rain (or depending where you are, snow) flying sideways and causing you to arrive at work looking like you've been doused with a firehose en route, the real enemies to comfort on board in the winter are moisture, humidity, and mildew.
As temperatures outside plummet and the skies grow dark increasingly earlier, the hibernating animal in all of us favors keeping the elements out. While there are a variety of ways to keep the cabin warm, it should be noted that warm air holds more moisture than cold air. When warm, moisture-laden air hits cold surfaces like windows, hatches, or the hull, it cools and gives up the moisture in the form of condensation. This moisture, if left unattended—and especially if it has seeped down into the dark recesses of lockers, or along cushions—is ripe for mold and mildew to breed, and those are two shipmates you definitely don't want on board.
There are a couple of solutions to the condensation problem. Heaters and furnaces can help keep moisture levels aboard down to appropriate levels. Heat produced by electricity is a dry heat, unlike heat produced by propane, alcohol, or kerosene. Liquid fuels release water vapor that can contribute to the mold and the this-doesn't-quite-feel-dry factor. What kind of heater you use will depend on where you are. If your boat is kept dockside and you have access to shorepower, electrical heaters are cheap. Last winter here in Charleston, SC, our heating bill came to about $7:00 a month; granted we only saw snow on the docks once and aboard our vessel we have a relatively small cabin space to keep warm.
Obviously, boat size will be a contributing factor to how viable this system is. Care should be taken not to overload power constraints. The dockside power supply may vary depending on the wiring, and shivering neighbors upstream from you can further complicate the issue when they crank their electrical heater as well. If you are running a heater through the 110-side of your electrical system, make sure the wiring is up to the task. Electrical heaters require serious amperage, and if you're running fancy equipment like water heaters, microwaves, or other apparatuses of creature comfort developed for use ashore, a bullet-proof electrical system with plenty of fuses and breakers is the one to have.
Onboard stoves or furnaces are another option offering unparalleled coziness for the wintering liveaboard. These produce dry heat, and sometimes actually require a pot of water on top to add humidity. A lump of coal can keep the fire going all night while the elements rage outside.
On the other end of the simplicity spectrum are diesel-fueled, forced-air heaters. Although your checkbook is likely to feel the hit these devices pack, in many ways they are optimal in keeping the boat warm and dry—providing you don't have to take one apart to fix it. These heaters use the boat's 12-volt system to move air, and take diesel from either tanks for the engine or a designated tank for the heater. Since heat rises, these are usually located low and use ducting to blow warm air throughout the boat while venting the exhaust outside.
I should mention that anytime you are using an open flame, oxygen is consumed and gasses like carbon-dioxide and carbon-monoxide are being produced. Regardless of whether you are going with the pot-over-the-stove route (which may require frequent trips for propane) or a kereosene heater from the hardware store (likely to saturate the air with moisture), ventilation is a must anytime combustion is used to produce heat. You should make sure that you can keep a hatch opened slightly, or have other means to allow oxygen in, like a dorade vent, at all times.
Having the right insulation can also pay off when the mercury starts to drop. Various strategies employed by those slip-bound for the winter have ranged from contact-cementing closed-cell foam to bare fiberglass to applying hardware-store-variety insulation via an adhesive or applying a layer of outdoor carpet to the inside of the hull. A roll of temporary insulation, which is a cross between aluminum foil and bubble wrap and is available at large hardware stores, attached to the hull kept our v-berth free of condensation last winter for the first time in years. The success of insulating the side of the hull depends on ensuring that air cannot get beneath the layer of insulation. A product called Dri-Deck has also brought good results when used underneath cushions. Drilling holes into cabinets and lockers is also another way to keep air moving, and keep the mold at bay.
Dehumidifiers are one of the most effective tools to use in the war against moisture. If you've ever been lost in a thick plume of steam while straining pasta, you know a bit about water vapor. Dehumidifers keep the air circulating through them and extract the moisture out of it. They do this by condensing the moisture onto a cold surface and then draining the resultant drops into a pan. The use of a dehumidifier can make the difference between sheets, blankets, and towels feeling dry and actually being dry. But make sure you keep your bilge free of standing water when you're running a dehumidifier or you'll simply be recycling that water.
Armed with one or more of the above strategies—and your favorite blankets—life aboard this winter should be a comfy one. Your land-based friends might think you're crazy, but come spring, you know they'll be back down on the docks helping you enjoy life afloat.