Re: 31 degrees tonight
31 degrees for a few hours usually won't freeze... much. True, water does freeze at 32 degrees, but it's going to take a while for the engine temperature to reach that low of a temperature, plus cool down the warm water inside to freezing. If it is for just one night, I would cover the boat up the day before with a tarp. The sun beating down on the tarp will warm it up a bit inside, then act as a blanket at night.
In my experience with RV's, the temperature has to be below 28 degrees for at least 24 hours for things to start freezing solid.
Always better safe than sorry, though. You might at least drain the water out from the block and manifolds, though, before covering it to minimize what can freeze.