rust in the bilge from a bolt...
I assume that the bolt in question is a keel bolt. Having rust and a leak at a keel bolt is not a good thing because the bolt will crevice corrode where it passes through the fiberglass. In the long run the only sure cure is to drop the keel, clean the joint, inspect the bolts, rebed the joint and put it back together. There are things that can be done in the short term to keep water out of the boat but that does nothing for the ongoing deterioration of the keel bolts.