If you are using a zinc anode the following things can cause it to loose its productivity. Using the boat in brackish or fresh water, exposing the zinc anode to air after it has been submerged in water, a poor bonding connection to the metal surface it is protecting.
The diver could use a stiff stainless steel brush, cleaned the complete anode of the oxidized surface, removed the inactive anode used, use a wire brush to clean the mating surface of the anode to trim tab plane, install a new anode. Clean the hinge area with the wire brush to remove the corrosion and oxidation that has formed at the contact points of the trim tab plane and hinge plate. ( crevice corrosion can form in theses areas. This can cause the hinge plate to have a poor bonding connection to the trim tab plane. The trim tab plane has the anode bonded to it at the attachment point. The hinge plate and transom mounting plate rely on the contact points of the tab plane to hinge connection to be a clean connection. There must be continuity from anode to plane surface and plane surface to hinge plate for the anode to protect.
My personal opinion is uses aluminum anodes. They are better for the environment, last as long if not longer than zinc, they can be used in salt, brackish and fresh water without loosing productivity.
