I've had a Marinico GalvanAlert "Shore Power Corrosion Detector" for a year, but I only got to use it briefly at first.  That was time enough to see how handy it is to have a power tester right in hand as you hook up your shore cable.  At minimum, a green LED will tell you that a dock receptacle is live.  Plus you'll get a red LED if the polarity is reversed, and two yellow levels of warning about stray current in the ground line, i.e. the stuff that can eat metal parts off your boat's bottom.  By now I've used the GalvanAlert, which costs about $140, a fair bit at a dock and even in a shed, and have seen how its steady monitoring can reveal shore power mysteries...

A couple of days ago, for instance, the GalvanAlert started warning about a "medium" galvanic danger in the boat shed, though it's hard to understand why.  The stray current is not related to anything on board, as the yellow LED stays lit when unplugged from Gizmo.  I also tried removing a variety of extension cords that yard workers have plugged into the same large service box, but the warning remains.  A well-informed person answered Marinico's customer service line promptly, I'm pleased to report, but, while reassuring about the actual galvanic danger (none), couldn't guess why a leak into the ground wire had suddenly appeared.  Could it have to do with the real ground thawing?
   Another mystery was the intermittent polarity reversal GalvanAlert twice alerted me to while Gizmo lay at a marina float last Fall.  Wayfarer has a fairly new and first class shore power system, and it was hardly being used at the time, but Gizmo's Paneltronics AC panel twice confirmed a reverse polarity situation and, in fact, had neatly shut down the shore feed because of it.  I might not have known what was going on, though, if it weren't for the GalvanAlert's bright red LED positioned right by the boat's main door.  In both cases, some combination of cycling the power post's circuit breaker, switching receptacles, or waiting a bit got things back to normal.  But I can tell you with some certainty that there's a least one 30 amp outlet that has the correct polarity 99.x% of the time, but gets reversed every once in a while.  Can anyone explain that?
   At any rate, I was already somewhat mystified by the complications of marine AC before experiencing these odd situations, and so I'm quite glad to have the help of a GalvanAlert.  Of course the real boat saver, possibly even life saver, would be if a nearby boat or a dock wiring mishap caused a sudden high current flow through the ground wire.  Apparently galvanic isolators can't help with more than about 1.5 volts, and the Marinco tech suggested that Gizmo's 10 year old isolator should be checked anyway.  (I found some instructions at the Yandina site.)  Of course another shore power danger to worry about is plug corrosion causing resistance, which then causes a boat fire.  Which largely accounts for the development of the interesting SmartPlug system, to be discussed soon.