I am in Florida, so I cannot help you with the time frame of when you should take the boat out, but I used to live in Louisiana and Mississippi and we would occasionally get a freeze or two over there. Only experience with cold was over there and I bought an engine compartment heater just to be safe. It is made by a company called Xtreme heaters and I love how the product works. A bit expensive, but it let me sleep well on those nights when it got really cold, knowing my engine wouldn't crack. I got the 450w. Let me know if you have any questions.
They automatically turn on when it gets colder than 40 degrees fahrenheit. If you have a plug, these heaters work really well. Just remember to set the heater under the engine but still in the engine compartment obviously (heat rises).