Boats typically rot from the bottom up. By the time you've got a weak spot on the deck there's usually soaked flotation foam and rotted stringers below. The foam and stringers contact the transom and rot that out as well. This is not in the least bit unusual for an old fiberglass boat. To repair it correctly is not particularly difficult but it is very messy, time consuming and the fiberglassing supplies aren't exactly cheap. Unless you really really like the boat, I'd scrap it. If it's got a good motor on it, find an aluminum hull to put the motor on.