In the development of renewable energy in China's coastal seas, the environmental risks must be assessed. Using the analytic hierarchy process (AHP) and reciprocal judgment matrix, in this study, we calculated indicators such as the high-value frequency, extreme value, average value, and climate change trend, as well as the integrated risk indices of three typical disastrous weather conditions, i.e., strong sea surface winds, big sea waves, and tropical cyclones. To help avoid the associated natural risks, we then used these three risk indices to comprehensively regionalize the risk in developing renewable energy such as wind, wave, ocean-current, and tidal energies in China's coastal waters. Research has shown that high-risk areas include the northern South China Sea, Luzon Strait, and the northern Philippines Sea, where tropical cyclones occur frequently. These cyclones trigger local strong winds and big waves, or spread energy through the Luzon Strait to the South China Sea, thereby indirectly triggering big waves. Moderate risk areas include the North Bay, central South China Sea, central Philippines Sea, the coastal waters surrounding Taiwan, and the central East China Sea, over parts of which strong winds tend to prevail but are not as risky as tropical cyclones. Low risk areas include the Bohai Sea, most parts of the Yellow Sea, the northwestern East China Sea, and the southern South China Sea, where each of the wind or wave indicators is very low, and the direct and indirect effects of tropical cyclones are negligible. We regionalized the wind-energy and wave-energy resources based on their indicators, i.e., effective wind speed frequency and effective time ratio of wave-energy development, respectively. In this paper, we present a comprehensive regionalization of both the resources and risks of wind energy and wave energy at sea.