External dependence of China’s ocean Industries

China hosts the world’s largest and fastest-growing ocean economy, yet it now confronts heightening external trade risks driven by global growing instability. To understand its potential risk from external shocks, this study evaluates the dependence of China’s ocean economy from both forward and backward perspectives and assesses the potential impacts of a U.S.A.–China decoupling based on a provincial ocean economy input–output table. The findings indicate that: 1) the forward external dependence (0.12) is higher than its backward external dependence (0.08), revealing that China’s ocean economy is more sensitive to fluctuations in external market demand than to disruptions in the external supply. 2) The external dependence exhibits significant regional and sectoral heterogeneity. While Guangdong and Shanghai show high external dependence, incorporating HHI concentration reveals that Jiangsu, Liaoning, and Shandong possess greater structural vulnerability. Industries such as marine shipbuilding (forward) and marine chemical (backward) are the most externally exposed industries. 3) The U.S.A.–China decoupling would significantly reduce total output of China’s ocean economy, particularly in marine transport and chemicals; however, scenario simulations confirm that global trade substitution can effectively mitigate a substantial portion of these losses. These findings provide policy insights for managing external risks and strengthening industrial resilience.