My own ship has a big fat lead keel encased in glass fibre.... the ship is a Union 36, with a modified-full keel. Now into her 30th year, the keel is zero maintainence, so far.... no visible corrosion, and no keel bolts!!!!
Cast iron is likely to be a good material too. Despite the association with steel (there is a lot of iron in steel !) cast iron really does have good corrosion resistance (witness raw water cooled Volvo motors... cast iron).
It is a rather brittle material though.