Since you don't use shore
power, do you have any other shore connections like cable TV, data cable etc.?
If there are no shore power connections the
electrolysis is being created locally on your boat. Are you in fresh
water? Magnesium is a high voltage zinc for use in fresh
water. In
salt water you should use normal zinc zincs. Magnesium anodes in
salt water are just self destructing "over protecting" the boat.
Make sure all the underwater items are connected to the same place, typically the 12 volt negative. Check that the exposed underwater metal on each item is connected to the negative and if it is not, then bond them together with a coppoer wire. The wire carries nearly zero
current so it only needs to be heavy enough to survive in the
marine environment.
Once you get all underwater metal connected together any zinc loss is due to different metals in the water and you will have to live with it.
The Hoda 2000 and its strange grounding "should" have nothing to do with your
electrolysis problem.