The current that causes electrolytic action usually emanates from a poorly installed electrical circuit or a bad grounded arrangement, on power tools or a radio for instance, or current leak due to damp conditions. Short circuit is also a common situation. Preventing electrolytic corrosion is a matter of good electrical installation. The wiring system should be insulated return (two wires) rather than earth return. A metal hull must never be used as the earth return. A battery switch should be fitted on the battery positive terminal and turned off when the boat is idle. Grounded (grounding) is required for safety if voltages are high, as when an onboard 240 volts generator is fitted or a shore supply arranged. Grounded in this sense is not to be confused with earth return. Earth return carries current; grounded or grounding involves a third wire which does not carry current. The earth or ground should consist of a sacrificial plate on the underside of the hull well away from the propellers.   Examples from a good and a bad ground connection:

Right radio connection

Wrong radio connection