"What?" you ask, "are you saying there are zincs on my water heater?" Well, in a word, yes. Or there should be.
Let me come at this from a slightly different direction. Have you ever wondered what the main difference is between 5 year and 10 year warranteed household water heaters? Zinc. All water heaters come from the factory with a zinc anode installed. The longer the warranty, the bigger the zinc. (You can, of course, buy new water heater anodes, should you wish to extend your water heater life beyond the warranty period.) As a boat owner who is familiar with the concept of galvanic corrosion, you will not be surprised that a system made of plain steel and which features bronze, brass and copper all in contact with each other needs some kind of corrosion protection for the steel.
water heater tank is made of aluminum. Although a zinc anode would protect it if it were filled with salt water, like for outboards used in fresh water, the manufacturer sells magnesium anodes for it.
I simplified things a bit with that opening sentence. In fact, I installed an anode in our water heater. The manufacturer shipped the water heater with *no* anode installed. Despite the installation of a bronze overtemp/pressure relief safety valve. Bronze in aluminum with no anode. Huh. Instead, the manufacturer requires that a handy plastic valve intended for draining the tank gets replaced with the anode. That made it easy to install, but it will make changing it next time difficult! There'll be no drain valve to use when that happens.
And, typically, I am remiss for not doing this sooner.