You guys are playing a big guessing game. Here in Northern California the nightime lows often drop down into the teens but daytime highs usually rise up into the 40's so the average is typically over freezing. Based on this I rarely worry about my boat freezing.
I have access to data recorders at work, one or two may even be on permanent loan to me. I have measured my motor compartment temperatures on a few occasions so I am comfortable with my boat outside being without being winterized.
There are several varieties of inexpensive temperature loggers used in the refrigerated trucking industry (not my industry). You might want to spend a few bucks, grab some temperature recorders and run some tests to determine what kind of temperatures your motor compartment is seeing. There are too many variables for any of us to say that your boat is save when average temperatures are down in the teens without looking at the data.
Here are some images to give you the idea.