I keep a remote reading wireless thermometer in the bilge and another that reads inside / outside the saloon itself.
Even when it got down 18 degrees outside for several days, the lowest I got in the cabin was 30 and the engine compartment to 34 degrees.
I keep a small heater set on 'freeze protection' and it keeps the cabin above freezing. To me it's just another excuse to stop by and visit my mistress during her winter sleep.