Vic,

After the prop is cleaned to the bare metal and restored, spray the prop and shaft with multiple coats of Rustoleum Cold Galvanizing Compound. A spray can is about $6 at Home Depot. It has very high Zinc content and nothing grows on it. You just wipe it off with a soft cloth when you clean the bottom. It is not a hard paint so it should not be burnished or cleaned with any abrasive pad. I put 6 coats on this season and it worked great.