As a marine renewable resource, ocean thermal energy conversion (OTEC) is gaining attention because of its sustainability and environment-friendly characteristics. The thermal energy potential in the open ocean has been well documented in the previous literature. However, regional potential near the coast is not easily estimated using methods and parameters for global estimation. This study estimates the thermal energy potential of the Aguni Basin in the East China Sea using a high-resolution ocean model. An additional simulation without tide is performed to clarify the tidal effect. The results show that two traditional methods overestimate the OTEC power density in the Aguni Basin, while two other approaches are able to improve the estimation. The OTEC power density in the Aguni Basin is much larger than that in the global ocean, largely owing to vigorous tidal mixing. Strong and sustainable power generation is possible near the northeastern slope of Kumejima Island as a result of the high OTEC power density within a short distance to the coast.