I have the 450w Xtreme heater. It now comes on at about 38 and shuts off around 46. The ad says it should be 40 – 50. I guess my temp gauge could be off a little. I am in Washington state and we had some pretty cold days and nights here a couple weeks ago and it kept everything nice.
I use a ceramic heater in the cabin which works well. Set on low it keeps it just above 40. My boat sits beside my house and when I am going to take the boat out I remove the heater since it is not mounted.