you might want to try to get a small heater in it, if you can get power. if not how about a few oil lamps, light em and let them run out of oil. another option is to get a mr heater and run that till out of propane. now it would dump lots of water vapor but what you could do is close the boat, with the heater running, come back a few hours latter
pump out the water then open the boat up to get the humidity out. if you have a honda genny or borrow one you could run that to run the heater till out of
fuel, then
pump.