Given you have a 3-way switch, disconnecting the battery sounds like overkill to me. Also, it means that your bilge pumps are disabled. IMO, bilge pumps should always be powered.

They are about $85 each but I recommend you fit double pole isolator switches to each battery bank as is required on vessels in commercial survey.

These switches isolate the positive and negative sides of the batteries which eliminates any possibility of stray currents. The care you have to take with these is that when the engine is running at least one of them must be in so that you do not open circuit the alternator.

Your auto bilge pump, if hard wired and insulated correctly should give you no electrolysis problems.

You will have a heavy earth cable to your engine and one way or another, via engine mounts, prop shaft etc the engine will earth to hull. That earthing may be intermittent which will cause problems therefore I recommend fitting a proper earth cable from engine to hull. Good if it is bolted to the hull just inside where one of the external anodes is.

www.whitworths.com.au/swch-bat-orange

THIS WILL DO

My 2c. (Battery electrolysis, not shore power electrolysis) is caused by the current flow from one of your battery terminals to the other via a mysterious route involving props, anodes radio grounds .... and across your joints of dissimilar metals. Disconnecting both terminals of the battery as the previous owner did certainly guarantees no electrolysis caused by ships battery as has been said. But technically speaking disconnecting one side should do it, no current would flow either. The current cant flow. Although if there was a damp salt coating on the side of the battery that was supposed to be switched off some current might flow. And electrolysis happens quite happily with a tiny current flowing over a long time.

I think the double pole battery isolator is a great idea and has two purposes. If you think about the circuit on the boat isolating only a side (ie switching off the positive of the battery with a big switch) will leave the negative side of the battery connected to wires running all around the boat looking for trouble at the worst time. If you isolate only the negative side (with a big switch) the positive of the battery will be connected to wires running all around the boat looking for trouble. If you cut off both the electrolysis cant happen as a wonderful side effect (unless the switch has a damp salty residue) but safety wise your wires cant cause any trouble even if they want to.

There is absolutely no reason to disconnect the negative battery terminal even if it is grounded to the hull. Even were it possible for a circuit to be completed the negative connection will in fact make the hull the cathode in an impressed current system.
It is not possible to overprotect/blister paint by hydrogen evolution using sacrificial anodes, many eons ago when alkyd based paints were used on steel hulls it was possible to break down paint coating by saponification due to the alkalinity in the vicinity of sacrificial anodes. Not an issue now with modern coatings.
it is always recommended to break the power leads to anchor windlass with circuit breaker. It is possibly for a poorly insulated positive connection to become an anode to the negative terminal cathode and corrode the positive terminal. Ie the anchor well is usually soaking wet with salt water providing the electrolyte for this system.
An AC ground should not be terminated to the hull, this will effectively make the hull an anode to the earth grid. Also it is possible to discharge a DC component via the earth when using ac rectified speed controllers (hair dryers etc).
There is a lot of jumbo jumbo and misinformation available on the Internet,

FRANT-An AC ground should not be terminated to the hull, this will effectively make the hull an anode to the earth grid.

should be - an AC ground - jetty - anything around the boat must be grounded
- the same potential is only way to avoid electrolysis.

The only way that an "isolated" boat can suffer from stray current corrosion is if there is a potential gradient in the electrolyte (water) from one section of the hull to another. It may then be possible for electrons to jump onto the hull and discharge (causing corrosion) from another due to being immersed in a different potential electrolyte. A similar corrosion can be observed with steel piling immersed in the mud zone. Generally adequate anodes n a vessel will mitigate any danger of stray current corrosion from such potential gradients, albeit with a higher anode consumption rate.

Charriot said..
in this case I would be worried to run shore power and No Ground, than
Safety switch would not safe ! unless there is a different arrangement put in place

An earth leakage safety switch will still work. ie if the same current does not flow through both conductors the safety switch will trip. A steel boat is effectively grounded to earth so no need to add a further cable to another earth. A fibreglass hull is effectively double insulated or you will be using double insulated appliances not connected to earth anyway.