RE: zinks going fast
I would ask the marine to trace, and would not start unplugging other boats.* The real concern is not volts but amps.* It only takes a couple of milli amps to for an electric shock.* As small as 0.01 amps.* Most multi meters do not measure amps so buy a volt and amp meter.* Start at your boats shore power box and test the water and the AC ground. That will tell you if its the water of the AC ground. A galvanic isolator is only good for like 0.04 volts which is not much. *Another resource is battery chargers and large amps draws like heaters, water heaters, stoves etc which can add up.* Every time we have a new neighbor I check for stray volts and amps.* The best is to let the marina check the other boats and you or an marine electrician check out your boat.
To protect you can buy Fish/grouper zincs that you can hang of the side connected to you zinc loop.* During the winter I have on hanging from the bow and one off the stern as they are easy to check.*