because zinc is cheap, and zinc does oxidize. And it the the electrical potential with dissimalar metals that marine engineers look at to determine which metals and coatings to use to reduce the effects of galvanic corrosion. From some of the research I've been doing with some water types you won't get galvanic corrosion, but you get bacterial and algeal corrosion. in the another water type(chemical make up) you get accelerated galvanic corrosion but no bio corrosion. There are a great number of things that contribute to corrosion. Even the distilled water from your Wal-mart may have a completely different ph than the distilled water from your local super market. Also maintaining a ph level between 7 & 7.5 is optimal for minimizing corrosion and preventing biologicals in water systems.