Ever since wooden ships and iron men became a memory and metals went to sea, we've been trying to keep our craft from corroding. Recreational boats suffer most, because professionals constantly combat galvanic corrosion; amateurs do not.
Dissimilar metals in sea water (or dirty water) become the anode and cathode of a giant, floating battery. A weak current flows as one metal..the anode..slowly erodes. Some metals are attacked more easily than others. They readily become anodic and deteriorate.
Here is a list of metals found around watercraft, with the most easily attacked listed in order: Magnesium, zinc, aluminum, iron, carbon steel, stainless steel, tobin bronze, yellow brass, copper, red brass, aluminum bronze, composition "G" bronze, admiralty brass, copper-nickel-iron alloys, nickel, and monel.
Magnesium is worst, Monel is best. Platinum is hardly affected at all, but they have yet to build lower units, props, shafts and rudders out of platinum. Because zinc is cheap and easily attached, it forms the backbone of the simpler corrosion-protection systems. It is installed as a "sacrificial anode" on boats. In the case of many outboards and most sterndrives, it is in the shape of a trim tab on the lower unit. It can be adjusted in angle to reduce the effect of engine torque on steering and because it is made of easily-attached zinc, it becomes anodic to all the other metals. It is cheaply and easily replaced. On inboards, zincs are installed as a collar on the propeller shaft or as a slab on the rudder, trim tab or strut.
Sacrificial zincs must be in full contact with the metals they are to protect, and unpainted. The world is full of neat but unknowing skippers who cheerfully paint their zincs, not knowing that are insulting the protective feature and rendering it worthless. For the same reason, your should not go out of your way to create a corrosive situation. Do not apply anti-fouling paint with copper or mercury compounds to aluminum lower units or aluminum hulls without a good insulating primer coat of zinc-cromate first. It is wiser to use the newer, more modern anti-fouling paints based on use of tin compounds; if you must paint aluminum. REMEMBER: DON'T PAINT YOUR ZINCS AND DON'T PUT COPPER OR MERCURY-BASED PAINTS DIRECTLY ON ALUMINUM!
Using sacrificial zincs doesn't stop the "battery" affect created when dissimilar metals are immersed in water. It simply supplies an artificial anode that you deliberately sacrifice. You can, however, "short-circuit" that giant battery by bonding your boat. It should be done when the boat is built. All metals aboard are inter-connected with copper strap or cable inside the boat, with a large capacity strap running down the keel, with jumper cables joined to metal cabinets of electronic or electrical gear, plus the engine(s), strut(s), shaft(s) and through-hull fittings. By connecting all these metallic items, you short out the flow of current via copper wire or strap; which interrupts the corrosive activity of the metals.
Cathodic control systems, such as the Mercathode systems seen on Sea Rays, actually introduces a weak counter-current in the water so no metal can be an anode and corrode. In essence, it is "charging" the imaginary battery. This cathode control produces a thin hydrogen film on the metals (charging any battery produces hydrogen gas). The film alters the difference in the corrosion potential. These systems are relatively expensive since platinum must plate the control anode.
Underwater corrosion is a way of life for boaters. If you feel that your boat is being subjected to a high rate of corrosive activity, have a certified marine electrician inspect your systems. He can offer suggestions on how to properly protect your boats systems.