Assuming you want to heat the entire garage to do your repair, I like Mark's suggestion but that requires 220v electric, not sure if you have that in your garage. I think to do safely what you are looking for, heat the garage for days unattended, you should look at some type of furnance system, and not just a portable heater. Which may bot be as cheap or easy as you are looking for. Something with a thermostat. Also being in WI, if your garage has an open ceiling to the roof, there will be alot of wasted heat too.

I would look into something like this http://www.mrheater.com/ProductFamily.aspx?catid=46 You wont be able to leave it overnight, or at least I wouldn't, but I have seen a contractor do it. When you go to work on the boat, bring it and a tank, fire it up. Depending on the size of the garage and heater, it may warm up quick. Especially if you move up to a kerosene one. You can also direct it to blow the warm are under the boat, from a safe distance, and the warm air will rise around the hull. Draw back it they are loud to be around.

Just thinking out loud here.