To improve the utilization rate of renewable energy such as wind energy, solar energy, and marine energy, and ensure the fresh water supply on offshore islands, reasonable capacity allocation of source, load, and storage in the microgrid is the key to island microgrid construction. Considering the unique characteristics of source and load on islands, aiming at the problems faced by the source, load, and storage capacity configuration in the island microgrid, an optimal capacity allocation method of source, load, and storage for offshore island microgrid containing marine energy was proposed in this paper. Firstly, the typical island microgrid structure was analyzed, and the specific source and load models on islands such as wave energy, tidal energy and seawater desalination were established. Then, according to the operating characteristics of source and load on offshore islands, an outer layer for capacity configuration optimization and an inner layer for operation cost optimization were combined, and the double-layer optimization model for source, load, and storage capacity configurations in the island microgrid was proposed. Finally, considering the power supply reliability and residential water demand, based on a coordinated dispatch strategy in the island microgrid, particle swarm algorithm (PSO) was used to solve the bi-level optimization model. Case study shows that, the proposed optimal capacity configuration method can achieve more reasonable capacity allocation of source, load, and storage in the island microgrid and provide the theoretical basis and technical support for the future microgrid construction on the offshore islands.