The surveyor's report in the claim file mentions that the boat's owner first noticed pitting on the shaft in the spring of 1987. Finally by 1989 the owner reported that he was unable to put the badly corroded outdrive in gear. The last zinc, it seems, had been placed on the outdrive in 1985, an eternity when two dissimilar metals are immersed in saltwater. Saltwater is a more effective electrolyte than freshwater, which means that galvanic corrosion takes place more quickly in saltwater.
Galvanic corrosion can even occur when dissimilar metals are joined above the waterline.
Saltwater washing over the fittings eventually destroyed the Zamac fitting and gasoline poured into the bilge. The boat exploded when the engine was started but fortunately nobody was hurt. Above the waterline, you'll want to avoid using Zamac fittings on critical parts like the fuel fill when a boat is used in saltwater. And whenever possible, avoid using dissimilar metals, especially metals that are far apart on the galvanic scale. If this isn't possible, which is frequently the case, you can often insulate one metal from another, for example, an aluminum mast from a stainless steel fitting, by using zinc chromate, polysulfide, a wood pad, or an inert insulating material like nylon or tufnol.