Marine renewable energy (wind energy, wave energy, and tidal current energy) has important application potential for the construction and development of uninhabited islands, however, it is necessary to assess the richness and stability of resources before energy development. In this study, a marine renewable energy integrated monitoring system was deployed on a small island, Dongluo Island, in the northwestern South China Sea. Based on the observation data over one year, the energy density and availability are systematically assessed. The results show that the renewable energy resources in the sea area around the island have a certain utilization potential, in which the average wind energy density is 81 W/m2 and the available frequency is 44.6%, the average wave energy density is 1.74 kW/m and the available frequency is 29.2%, and the average tidal current energy density is 32.4 W/m2 and the available frequency is 18.4%. All three types of energy propagation directions are relatively uniform. In addition, because the seasonal variations of the intensity and stability of the three types of energy are different, the three energy resources show obvious complementary advantages. Although the available frequency of single energy is relatively low, the available frequency will be significantly improved by comprehensively superimposing the three resources. Therefore, for the energy development of the island, it is necessary to integrate and optimize the allocation of different types of marine energy resources.