Boats have stainless steel, bronze, iron and brass under the waterline and lots of 12 volts running inside that would love to leak out broken wires, bad connections and bad grounding. Now include 120 volt shore power, also of doubtful integrity and who knows what kind of surprises are going to take place under the waterline.
Galvanic corrosion is when two metals are immersed in sea water, which acts as an electrolyte. A galvanic cell is formed and current flows from the anode (the less noble metal which corrodes) into the electrolyte, and from the electrolyte to the cathode, the more noble metal. In other words, if you don't protect the metal in, on and under your boat they will soon corrode and will eventually need replacing. When current starts to flow between two metals one of them will pay the price.
It doesn't matter if the current occurs naturally or accidentally because of current leaking to ground from the batteries or shore power, in any case the least noble metal in the circuit will deteriorate and protect the others.
Zinc is the least noble metal readily available to boaters. By attaching zinc's strategically in places like out-drives, rudders, shafts, you can prevent damage from electrical flow.
The idea behind using zinc is when galvanic corrosion occurs the zinc will act as a sacrificial metal because of its low nobility. Therefore, the attached zinc's will deteriorate and protect your existing metal equipment.
When you attach a zinc, be sure the metal underneath is clean and bright for the best possible contact. Tighten the bolts as possible, then seat the zinc with a couple of raps from a plastic hammer and tighten again. This will make sure the zinc will stay in place.
If the internal bonding circuit of your boat is broken or incomplete, all the zinc underwater might not help. Check through the bilge and make sure the green bonding wire is attached to all metal through-hulls and running gear. Some boats have a wide copper strap instead of a green wire. Make sure the strap or green wire is properly grounded to a good ground source.
If your bonding circuit is good but you're still using lots of zinc's, check for broken wires, clean all bonding connections, incorrect installation of battery charger, electrical equipment and ground connection from the battery to your engine.
Some engines also have zinc's to protect them from corrosion. look for square plugs with the letter Z engraved in their tops. Refer to your engine manual to determine how many are in your heat exchanger or engine. Remove each zinc with an open end or adjustable wrench, than tap it firmly against something solid to knock off the whitish crust of dead metal. If more than half the zinc is gone, replace it and keep checking at regular intervals.