A galvanic ground isolator adapter protects against galvanic corrosion on a boat when a boat power system is connected to a shore power system. A ground isolation assembly in the ground connection between the two system grounds prevents current from passing between the two system grounds when the voltage difference between the two is low. When the voltage difference exceeds a preset value, the ground connection is established. The adapter is entirely contained within a rigid housing with a male plug at one end and a female socket at the other end. In this form it can be simply connected to a power cord that is normally used in connecting the shore and boat power systems. It may be provided with a locking mechanism that prevents it being removed once it is attached. It may be supplied with a sonic alarm to indicate when current is flowing in the ground connection.