Had the same prob afew years ago (my boat is berthed in brackish water) coz zinc ones tended to get coated with a layer of white chalky substance which prevented them from functioning to their full potential.
Spoke to MG Duff (theyre lousy at e-mail) and they recommende aluminium.
worked well, protecting outdrives, but it has to be the correct grade and v. expensive (2x price of zinc) but still cheaper than new outdrives