More on Zincs
B&B -
This is a complex topic, especially as we wander into the area of unintended consequences and stray current. And I'm by no means a professional in this area, I just read a lot.
Let's start with zincs - the purpose of zincs is to avoid electrolysis from dissolving the underwater metal on your boat. Remember the high school chemistry experiment with the two dissimilar metals in a salt water solution and a wire connecting them together? Current flowed (electrons moved), the less noble metal dissolved and ions of one metal moved over and deposited out on the other metal. They flowed from the anode to the cathode. That's how batteries work, that's how chrome plating works. For a battery it is producing current, in the case of chrome plating they induce a current in order to force the chrome ions to plate something.
(Disclosure - I really enjoyed my high school chemistry class, and not just because I got to burn things).
Well, you don't want to make an anode out of a folding prop, or your through hull fittings - you don't want them to dissolve. So instead you bond them all together and attach them to zinc anodes under water. Then if something is going to dissolve, it's the zinc, which is way down on the least noble end of the electro negativity chart. They are sacrificial anodes, you expect them to be replaced every year or two as they sacrifice themselves to save your prop and through hulls.
The electrolysis will happen faster if the current is not just what you get between zinc and bronze on the electro negativity chart, its is much worse if you have a voltage difference between different underwater metal parts of the boat, which is why they are all bonded together with those big green wires. Basically, if there is a voltage difference, you are sort of zinc plating your brass parts, and when the zinc is gone, you are brass plating your other brass parts, because there is always going to be one metal which is less noble than another (and brass is an alloy that varies from one maker to another)
Ok, so back to your electronics.
I don't believe that marine electronics requires a true earth ground, except for Single Side Band radio, which for sure should have a ground plate of some sort. I think (and I'm not an expert here, check and see what is in Caulder's book) most people do not have a special ground for their marine electronics; they just have plus and minus. Some marine electronics manuals say 'wire straight to the battery' which is just their way of saying 'get me a good low noise source of electricity. It's really a bummer to see a nice electrical panel and a bunch of wires attached to the positive post of the battery for the radio, radar, GPS, radar etc.
About the marina electricity - Boats that have a shore power cable are supposed to run the shore power through an isolation transformer - that gets the 120V AC power into the boat without a physical connection of the boat wiring to the marina wiring. The transformer lets the AC from the marina into a transformer that forms a magnetic field and which causes an electrical field on the other side of the isolation transformer and presto - you have 120v AC inside the boat without a direct electrical path. You do that because you don't want to have a ground leak on the marina put any kind of voltage on the boat that can then leak into the water.
Again - I might be wrong here - this is just what I've read - and AC current is dangerous stuff, especially around water. There have been documented cases of people being killed while swimming near boats plugged into 110V AC with current leaks into the water.
So what's this mean to you?
If you have a real shore power cord, I would hunt around to see if you have an isolation transformer, and if you don't you might consider getting one.
If you are just bringing a 110v extension cord aboard and plugging it into a battery charger, I would check with the battery charger instructions to see if there isolation between the 12v and the 110v.
The one thing I'm not clear on is the 'corrosion on your interior electronics' . That could be a bunch of different things. Boats that have been wired with non-tinned stranded automotive wire can have that wire corrode just by being in a damp salt air environment. I had a #3 cable (and that's a big cable) go completely belly up this summer just from internal corrosion of the copper causing the resistance to increase. Same thing goes for connectors - automotive connectors don't last forever on a boat - the environment is hard on them, and they show this by failing. Yes, I've used them, but I'm not proud of it, and as I find older connections failing (from the previous owner) I'm getting more serious about decent connectors.
And if you have any connectors that are soldered - well even flux in electrical solder seems to be an invitation for corrosion on a boat.
Maine Sail (a regular contributor here) did a great write-up on crimped connectors and made a believer out of me.