Comparatively speaking, seawater is a pretty good conductor of electricity. So all the underwater exposed metal parts of your boat are essentially electrically connected to all the underwater exposed metal parts of any other boat or boats within a reasonable distance. (A conductor's ability to pass electricity is greatly reduced by distance, and that holds true whether its wire or water doing the conducting.) So think of these nearby boats as being in contact. It's somewhat akin to the example of dissimilar metals that I mentioned above.
If you thought the corossion from dissimilar metals was easy to fix, the stray currents that threaten your boat's metal parts are even more easily handled. You've already taken the first step by bonding all of the metal components of the boat. Now, assuming that your electrical wiring is installed according to the National Electrical Code, the DC wiring is grounded to the bonded metal system created by connecting everything metal together. The AC wiring must also be bonded to this system to assure the proper operation of the overcurrent devices that protect crew and equipment aboard.
When shore power is connected to the vessel, the possibility of stray current electrolysis corrosion could exist if a ground or neutral faults to a current-carrying conductor ashore or on another boat, and ‘leaks' an amount of current too small to trip a breaker or blow a fuse. This tiny current flow may not be detected, and could exist for years with no damage or indication at the point of the fault. Electrical outlets that are wired incorrectly can create reverse-polarity, and this is commonly part of the problem. Such tiny current flows that lead to electrolysis may also be present when partial faults in the ground wires exist. This condition is avoided by doing three things:
We now have protected the vessel from corrosive destruction caused by chemically generating its own voltage (dissimilar metal corrosion) or conducting stray voltages at your marina. By implementing these fixes, you'll have a head start with lightning protection, which I'll discuss in my next article.
Onboard Electrical Guidelines