Zinc for aluminum or salt water: If you are a saltwater boater, you should install zinc or aluminum anodes to prevent galvanic corrosion on the engine and underwater parts of your boat.

Magnesium for fresh water ONLY: Since fresh water is much less conductive than salt water, magnesium anodes are the best choice because they’re more active (less noble) than zinc or aluminum so they will protect your engine parts more effectively. Caution: do not use magnesium anodes in any application other than fresh water because they will corrode rapidly, exposing your boat and engine to possible damage.

Aluminum for brackish water: More active than zinc, aluminum anodes are a good compromise where fresh and salt water mix, such as the brackish water of river deltas, or if you use your boat in a variety of water types. Zinc is too passive for brackish water, where it gets covered in a zinc oxide film and becomes inactive. Magnesium corrodes too rapidly (due to the water’s salt content), leaving your boat without adequate protection.