If the shore power cord ground is connected to the battery negative inside the battery charger (and this connection should be made for safety) then you will need a galvanic isolator in the shore power ground to stop electrolysis currents. Theoretically you could disconnect that ground and stop the electrolysis but that is dangerous. Although the chance of an internal short between the AC power and the DC output is very unlikely, the consequences can be very catastrophic if it kills someone. You can find Galvanic Isolators for about $100 and up.

If you are getting the electrolysis when there is no shore power connection, then you have different metals on the motor and/or the boat that are creating local electrolysis currents. There is usually not much you can do but things like switching to a non metal prop can sometimes help. To save your expensive motor mounted zincs and the difficulty of replacing them, buy a zinc fish. This is just a big lump of zinc on a wire that you hang in the water and ground to the engine. It will attract much of the local (or shore power) electrolysis and is much easier and cheaper to replace. You can pull it out of the water easily when using the boat.