In this study, we utilize the advantages of offshore wind power resources in coastal areas to make up for the shortage of freshwater. At the same time, freshwater can be used as raw material to supply hydrogen energy. An operation strategy considering power and water input for an electrolyzer is proposed and an electrolyzer variable efficiency model under optimal operation mode is also proposed. Considering both energy benefits and operation and maintenance costs, this paper sets up the wind–hydrogen–water power grid system optimal operation model. Simulation analysis is carried out from the aspects of economy, accommodation, uncertainty of impacts of the reservoir capacity, water satisfaction, and so on. The results show that the accommodation rate reaches 98.2% when considering the proposed operation strategy. The average daily operating cost of the system reaches USD 3.9 × 10⁴, and the strategy and model have good economic benefits and practical significance.