The anodes on your outboard engine are just like any other maintenance part: neglecting them may not hurt today, or tomorrow, but you can bet it will most assuredly do damage in the long run.
Your engine’s sacrificial anodes (or zincs, as they are often called) are designed to protect the motor’s other submerged metals from galvanic corrosion. When a boat is sitting in water – especially saltwater, or when hooked to shore power – the vessel’s metal parts become a battery of sorts, with minute amounts of current flowing through them. The laws of physics dictate that the softest, most reactive submerged metal in this system will dissolve over time. If your anodes are significantly worn or missing entirely, galvanic corrosion will attack your lower unit or aluminum propeller, which can cause costly damage.
This Mercury Quick Tip video will show you the location of the anodes on your Mercury outboard and demonstrate how to change them. Remember, anodes provide less and less protection as they dissolve, so be sure to change yours when they are 50 percent consumed.