Put two different metals in seawater and connect them, and the less noble one corrodes. An anode is a lump of metal chosen to be the least noble thing on the boat, so it goes first. Zinc was the usual metal for so long that many people call every anode "a zinc".
Martyr Anodes matches the metal to the water: zinc or aluminium in salt water, aluminium in brackish water, magnesium in fresh. It advises replacing at 50 percent worn or at least once a year.
What a surveyor checks
Every anode on the haul-out: shaft, hull, rudder, trim tabs, saildrive or sterndrive leg, and the ones inside the engine. How much has wasted, whether each is tight, and whether it's painted over. Then the metal it protects.
The anodes tell a story. Wasted fast means something is eating them, often a missing galvanic isolator on shore power. Untouched after a season can mean it isn't connected, so it protects nothing.
In the report
"Shaft anode about 70 percent wasted. Replace at this haul-out with zinc or aluminium for salt water." See depleted anodes.