Electrical, Corrosion and Electrolysis topics and articles.
What does your Capac read? It should always read between 0.90 to 1.05. BTW, It should not change when the boat is plugged in to shore, if it does, you have a problem with the isolator being connected.
I think 6ft of zinc (anode) is too little, anywhere. How often do you take the boat out of the water? It should give you an idea of how fast the zinc is eaten up. If your zinc is fouled (by barnacles or algae, it will drop in effectiveness. By zinc, I don't mean the metal zinc, I mean the proper anode material for these boats, see above.
Some say you can't overzinc a Marinette. I don't agree, Interlux told me that the Trilux Antifouling is attacked, if the zincing is too high, but I pull my boat every year, and redo the AF. (I don't know if TBT is affected). Magnesium may overzinc a boat in any but the freshest (softest) water.
I tried to use less zinc in the Potomac, and finally had to put 2 6ft peices on the keel and spilt up a third piece on the rudders, the swim platform and trimtabs. I finally got it down to a consistent 0.95-1.0 reading on the Capac.
I don't like the impressed current systems for these boats. Capac (US Filter) still sells them, but they require lots of careful maintenance.
In 2001, a new galvanic electrode (CN-2) for the CAPAC meter was replaced. My yard sandblasted the silver off. Arg! It strips the sliver (Ag/AgCl) from the coating. It cost them $200. These should last forever on an Marinette. In 2002, I replaced the push to test meter for $225.
A CAPAC meter is a voltmeter to ground, you can test with a real meter.
Best of Luck
BTW: Did you know immersed wood is also affected by overzincing and electrolysis effects? Put a barrier between wood and aluminum if there is a chance of continuous wetness.
Did you know - The average marina ground will drop a boat without a isolator by about 0.2 volts. No, it's not the marina at fault, they have to protect you from shock. The metal on the marina ground circuit causes the drop. If you connect to shore, you need an isolator!
If it's more than about 0.3 volts, the marina should be checked.
BTW : Tell those outdrive friends with Mercruisers to use a Martyr III (high magnesium) anode in fresh water, or a quality zinc (one that contains some Aluminum or is Aluminum-Indium) in saltwater.