we bought an auctioned swift-mud airboat a few years back. when cleaned up, the floor was really bad in spots. we cut out the spots and welded in patches. luckily the corrosion was not under the runners. made a fine boat.
Zincs will help with galvanic corrosion but not with chemical degradation. I fixed a neighbors aluminum jon boat that had a corroded area caused by a bag of fertilizer left in the bow. If you're considering buying the boat and the damage is caused by galvanic corrosion from salt water, the damage could be extensive…all the area that was exposed to the salt water electrolyte could be affected. It may still be a good deal if the rest of the hull and engine are good…and the price is right.