The keel bolts on a Beneteau First screw into the keel. While the heads may appear wasted, given the snaps you have posted, they are not unusual for the yacht's age. A couple of wacks with a maul on the top of the bolts to dislodge the larger rust and little work with a wire brush chucked into your drill will clean them up well enough for you to loosen and pull the bolts. That can be done one-bolt-at-a-time in the water, the process recommended by Beneteau, or, shoreside during a haul-out if you're the nervous type. If you will PM me with your email address I will forward copies of Beneteau's procedures for replacing the bolts. Once the new bolts are in place, they should be thoroughly coated with a moisture barrier as described in their literature but it's not a big deal. You'll do fine.
"It is not so much for its beauty that the sea makes a claim upon men's hearts, as for that subtle something, that quality of air, that emanation from the waves, that so wonderfully renews a weary spirit."