Go to homedepotlowes or ace and get a couple of these.
Put a 40 - 60 watt incandescent (you want the heat) bulb in them. Clamp them in the cabin preferably near water lines. Open all cabinet doors. This should suffice for now. Get an ER heater. Better safe than sorry. This may not be the last cold winter.
[URL="http://www.foxnews.com/scitech/2010/01/11/years-global-cooling-coming-say-leading-scientists/"]http://www.foxnews.com/scitech/2010/01/11/years-global-cooling-coming-say-leading-scientists/[/URL]
Be VERY careful using incandescent bulbs for heating like that. This was the norm for many many years, but in todays
litigious world, it is a big no-no. Your insurance and marina will not cover you if an accident happens using this method. If the fixture comes loose for any reason, drops and is resting on fiberglass or other flammable material, your screwed. Get the heaters like everyone else said.
Their are only 2 types of heaters that are approved for use in your engine room. They are made by Boatsafe and Xtreme. These are the only heaters that are approved with your boating insuarance and your marina for use in the engine room. A ceramic heater in the cabin can be used, but is frowned upon for use while the boat is not occupied. Don't kill the emessenger, just letting everyone know the "correct" and legal way.
Personally, I use the Boatsafe in the engine room, and a ceramic in the cabin. It is a constant 41 degrees in the engine room, and 60 degress in the cabin.