The Ship's Anodes
The only sure thing about boating is that trouble free operation will be the result of a hundred small fixes in obscure areas.
One critical area is protected by sacrificial aluminum anodes that corrode away to save the metal boat parts that they are attached to.
Sacrificial anodes are specifically designed to protect parts of your boat machinery that are underwater from galvanic and electrolytic corrosion, both caused by electrical currents in the water that the boat is sitting in. The boat's metal parts are the "terminals" of a battery whose electrolyte is the surrounding water.
These anodes ''sacrifice'' themselves by being "eaten" away by these currents, like battery plates.
Internal anodes are inserted into the engine to protect the coolers, and outside-mounted anodes protect the rudders & props.
Galvanic corrosion causes major damage if not controlled. Electrical currents in the water are everywhere but are particularly strong in marinas or docks where electrical cords droop into the water.
The picture on top is of a new pencil anode which protects the engine. The second shows a pencil anode after a season of cruising.
The last two pictures show an aluminum plate on the outside of the hull, exposed to the water, which is electrically connected to the engine metal via through-hull bolts. By the end of the season, this plate is partially eaten away by the electrical corrosion current.
Eventually these sacrificial metal anodes will disappear, molecule by molecule, so they need to be inspected and replaced when they reduce to about 50%.
Note: For more information, see "What is galvanic corrosion and how can it be prevented?"