Not trying to scare you, bu the maximum expansion for freezing water is right when it first crystallizes at 32 deg (or some temp very close to that) after that it acts more or less like any thing else and contracts as it gets colder. So 31 degrees will do as much damage as -20, just takes longer to freeze.

water temp of the lake say 50-60ish boat sitting 2ft off the water i would say not a problem. if it went to 22 all day and night thats a different story.
later

Thanks for all the posts guys.! I pulled it out and brought it back and put it in the garage.... I'm not a gambler, so it's all warm and happy now. Took it out for a lil bit before I trailered it. Boy do they run good when it's cold.!:p

SORRY TO HEAR ABOUT YOUR COLD WEATHER.HERE IN HARRISON B.C. ABOUT 1 HOUR EAST OF VANCOUVER B.C. "CANNADA" IT HAS BEEN 90 DEGREES AND YES IT IS OCTOBER 2 WHEN I AM WRITING THIS POST.UNFORTUNATLY I HAVE BEEN CURSED WITH THE DEAD BOAT SYNDROME DUE TO MY BOAT GETTING THE URGE TO INGEST AN EXHAUST VALVE.I AM SURE IT WILL RUN GREAT NEXT FALL WHEN IT IS FREEZEN OR RAINING.GOTTA LOVE BOATING.