From FisheriesSupply.com:
“Which Metal Should I Choose?
Saltwater: Aluminum anodes are more active, protect better and last longer than zinc anodes in saltwater – a win/win situation. Magnesium anodes are so active in this highly conductive liquid that they completely corrode in only a few months, resulting in prohibitively high replacement costs. While zinc has been the traditional anode for use in saltwater, it doesn’t provide as much protection or last as long as aluminum.
Brackish Water: Aluminum anodes provide superior protection here. They do not suffer the fast corrosion rates of magnesium, and protect better than the less active zinc.
Freshwater: Magnesium is the clear anode of choice. It offers superior protection in this low conductivity liquid. Zinc anodes are not suitable for use in freshwater because they build up a hard, dense coating over a period of months – rendering the anode less effective. Aluminum alloys offer some protection in freshwater but not as much as magnesium – so aluminum would only be recommended if your boat is intermittently in fresh water.”
We switched to aluminum anodes for most applications on the boat.