Any earth problem on any boat or shore will increase the voltage in the common earth cable, adversely affecting your boat by dissolving your anodes at an alarming rate. In another scenario, if you (for example) have a zinc anode on your boat, but the boat beside you does not, then your boat's anode will protect all the structures and boats around you, resulting in a dramatic zinc loss, which will result in an expensive lift out to replace your anodes.

This is where a galvanic isolator comes into play. The isolator ensures you can maintain a good earth link with the shore, but any stray currents will be prevented from coming up the earth line and damaging your boat. 

Other models available:
Sterling 16A Galvanic Isolator.
Sterling 50A Galvanic Isolator.

Specifications:

Amps: 30.
Dimensions (L x W x D): 120 x 100 x 90mm.
Weight: 1kg.