i live on Lake Norman in NC and we see mid 20s all the time (sometimes into the teens) and i rarely even turn on a heater in my boat. i leave water in the fresh water tank and the engine is not winterized. Sitting on the lake water helps protect it. As long as there is not a real sustained cold the little bit of heat from the unfrozen lake should keep you safe. I can't speak for the out board engine. i have an inboard diesel.