Kari,
If the boat is in the water, simply being that close to a large heat source 
should keep everything from freezing. That being said, taking precautions can 
save a lot of grief later on. Water tank.....empty and put in some RV 
antifreeze, the nontoxic kind. Porta potty or marine head should be 
emptied....dry. Personally, I put two bottles of cheap vodka in the water 
tank and run it until I have vodka coming out of the faucet in the galley. It 
makes for great entertaining come the warmer weather. I have a party to empty 
the tank. :) Throw in a little bloody Mary mix and your set for a couple of 
hours of great sailing.
Battery should be charged fully. A run down battery (no charge) can freeze 
and crack the casing.
Most motors run dry when they are taken out of the water, unless there is a 
thermostat on it. It depends on the make, size and age.

Rummy in SC