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.
2003 Outback, Assault EFI, (6) Polk Audio DB651s, JL Audio M6600 amp, JL Audio M1700 amp, JL Audio 10w6v2 subwoofer