Current then flows between the metals. Electrons comprising that current are made by one of the metals actually decomposing, shredding itself — in the form of metal ions — into the saltwater.
This is called galvanic corrosion, and if not prevented by adding a third metal to the circuit (that sacrificial zinc anode), your submerged metal outboard will be quickly eaten by corrosion.
"I cannot emphasize enough the importance of maintaining the anodes on your boat," outboard mechanic Gabe Dominique said. "If an anode is missing, crusted over or a portion of it is corroded away, the metal parts of your outboard motor that it was installed to protect will become the target and quickly begin to rot away."