YOu can try the Home Centers if they carry a liquid spray that will 'loosen' the bolt and rust so you can turn it. If they don't carry it, I know a decent Automobile Parts Store will. There are LOTS of bolts on cars that get rusted into place.
You didn't say if the bolt had a screw, or if the setup was all metal, or if it was a Lag Bolt screwed into plain, old wood. As Carol suggested, heating the bolt causes it to expand. The expansion may 'compress' the wood fibers just enough, so that when the metal cools, the bolt will be a little 'loose' thus allowing it to turn.
RE: Liquid Wrench - I don't think it will work if the bolt gets wet. DON'T hesitate to use it a couple of time. Just read the directions on the can BEFORE you leave the store to see if you can use it. At worst, visit a Marine Supply Store (or a Boat Shop) for more help. They may have something you can use if the bolt is getting 'wet'.