Otherwise, plating would be cheap and not as expensive as it is. Electrolysis only occurs on boats when there is a ground fault and it is leaking current, usually from bad wiring. Small current leaks are commonplace and are rapidly dissipated in surrounding metals without doing damage. Larger current leaks, as when a significant part of a large conductor has a ground fault, can produce very damaging corrosion. People often see heavy pitting on propeller blades and rudders and attribute this to electrolysis when, in fact, it is usually the result of yet another form of corrosion, erosion corrosion caused by fast streams of water laced with air bubbles.
Erosion corrosion is strictly a mechanical form of corrosion.
This is a from of corrosion caused by a current flow generated between two materials with greatly differing electrical potentials. Carbon rubber mated against aluminum, for example, is a disastrous combination that will destroy aluminum very quickly. On wooden boats, galvanism is rarely a factor except in small isolated areas. When two dissimilar metals are mated electrically, they generate a very small amount of current that has little power. That means that the electricity generated has little capacity to flow very far before being dissipated.