This 32A galvanic isolator from Victron prevents electrolytic corrosion by blocking low voltage DC currents that enter your boat via the shore power earth wire. These currents can cause corrosion to underwater metals on your boat, such as the hull, propeller and shaft. 

Inside the galvanic isolator are two diodes which are connected in antiparallel fashion. When connected this way, the diodes allow current in both directions, but only above a threshold voltage of 1.4 Vdc. The forward voltage from the galvanic isolator is higher than the potential difference between metals - as a result, this voltage will not allow conduction, and as such, the galvanic isolator will prevent any electrolytic current.

However, be aware that if there is a higher error voltage in the AC circuit, the diodes will allow current through, and the residual current device will break the circuit.

Other models also available:

Victron 16A Galvanic Isolator Unit.
Victron 64A Galvanic Isolator Unit.