Electroless nickel plating (ENP) was performed to prevent the cavitation erosion damage of gray cast iron in the marine environment. The effect of the changes in the pH of a plating bath on the ENP coating characteristics, corrosion resistance and the cavitation erosion damage behavior in natural seawater were examined. The results showed that the pH of the plating bath had a significant effect on the plating rate, crystallinity, composition, and microhardness of the ENP coating. The corrosion resistance of ENP coating was greatly affected by the plating rate and the microstructure of the substrate. The cavitation erosion damage behavior depended on the crystallinity of the ENP coating. Once the substrate is exposed by the cavitation erosion of ENP coating, it is accompanied by galvanic corrosion, which accelerates the damage. Furthermore, the cavitation erosion damage mechanism of ENP coating was investigated.