To meet the current CE standard (ENSIO 13297) the ground of the AC shore power must be connected to the ground of the boat. Connecting the ground wire of the AC-shore power to the metal parts of the boat will result in galvanic corrosion. It can occur on any boat as soon as a metal part (shaft and propeller) is in contact with water. The corrosion will quickly dissolve your zinc anodes and attack the shaft, propeller and other metal parts in contact with the water as soon as the boat is connected to the AC shore. Installing a Galvanic Isolator / Zinc Saver will maintain a good earth link with the shore, but prevent stray currents coming up the earth line and damaging the boat. Protected to IP65.
Not much to say, just a good product