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It's known as "cathodic protection". You connect a controlled piece of a more reactive metal onto the structure of the boat, IMMERSED IN THE WATER, making sure you have good contact with the electrical bonded system of the boat.
Classically, it's a fist sized piece of zinc bolted to the hull, and a wire led from inside the hull to all of the through-hulls. You probably have seen all those wires running from through hull to through hull inside the boat. That's the bonded system, and it connects all those expensive bronzes. Some owners drop a wee zinc "fish" over the side, and lead the other end of the wire back inside to the bonded system, or sometimes to the engine block, or whatever they want to protect.
Zinc is more reactive than the through-hull bronzes, and more reactive than iron if that's what it's wired to. Immersed in water, the effect is a very weak battery, where the zinc slowly reacts away. As it reacts, the zinc is always a few millivolts more positive than the rest of the system, and so the through hulls are a wee bit negative with respect to the zinc. This has the highly desirable effect of radically slowing the corrosion of the bronzes.
At intervals, the zinc is renewed. You know by looking at it.... it looks "crumbly". Since the zinc is sacrificed in this way, it is sometimes known as a "sacrificial anode".
In fresh water, zinc does not work so well, and some owners use magnesium. You see it on outboards sometimes. Magnesium is more reactive. Corrosion is much less a problem in fresh water though. Fresh water is good stuff, but it does freeze, as I learned expensively last year.