You probably already thought of this...but...you don't have to haul or dive to experiment. Tie the magnesium to a stainless wire and clamp it to something like a handrail and see what happens to your reading. Some of our folks use a zinc guppy (anode shaped like a wittle fishy) to goose their Capac reading. If it helps, then you can leave it that way when you're docked or permanently install it somewhere.

With new anodes, you should be above 1.0. Check whether the capac meter is reading correctly by using a digital voltmeter on the back of the capac meter and compare readings.

Motor away from the marina, turn off all dc breakers, and see what the capac reads. If it is still 0.75 or less, check for grounded shafts or rudders. While the shore power cord is disconnected, measure the resistance between the white ground lug and green ground lug on the shore power connection on the boat. Make sure you don't have an inverter on while you are running this test. You should have at least 25,000 ohms resistance. If lower, then there is somewhere that someone has connected the white ground wire to the green ground wire in the ac wiring. Or, your galvanic isolator is bad.

Now, turn each dc circuit on one at a time and see what the capac meter reads. Then check capac when you get back to the marina before and after attaching shore power. I found that my cable tv connection was delivering 0.3 volts to the hull! I had to ground the cable tv sheath so it would not energize the hull.

Paul