This study evaluates the accuracy of ERA5 10-m wind speed and significant wave height (SWH) using one-year hourly observations from 16 buoys in the Bohai and Yellow Seas. ERA5 agrees closely with the buoys (wind: Bias 0.04 m/s, root-mean-square error (RMSE) 1.40 m/s, scatter index (SI) 0.24, correlation coefficient (r) 0.90; SWH: Bias 0.11 m, RMSE 0.23 m, SI 0.30, r 0.94). Errors decrease offshore, and the control is offshore distance rather than water depth: a 2.5-fold depth change at fixed distance alters wind RMSE by 0.02 m/s, whereas a doubling of distance at fixed depth alters it by 0.22–0.24 m/s. Wind Bias has a marked diurnal cycle whose amplitude decreases offshore and reverses seasonally; SWH Bias stays positive but weakens in summer; both variables overestimate weak and underestimate strong conditions. The absolute errors of the two variables vary out of phase, so their correlation cancels (−0.40 to 0.02), whereas the relative errors remain strongly coupled (0.59–0.80). Above 13 m/s, ERA5 underestimates wind by 11% while overestimating SWH by 6%, an inconsistency that locates the high-sea-state error in the wave model rather than in the forcing. These quantified errors constrain design-wave estimation and Bias correction in coastal engineering.

Significant wave height (SWH) is vital for maritime safety and oceanographic research. This study systematically evaluated the accuracy of the multi-mission along-track SWH product distributed by Copernicus Marine Service (CMEMS) with a focus on China’s coastal waters. Validation used 2022–2024 in situ data from 30 coastal operational buoys. A spatiotemporal collocation of 25 km and ±30 min was adopted to balance sample size and matching accuracy. The SWH product from CMEMS showed good performance with an overall bias of 0.10 m, root-mean-square error (RMSE) of 0.19 m, correlation coefficient (CC) of 0.98, and scatter index (SI) of 0.13. Spatially, errors tended to be lower in the south, offshore, and in deeper waters. All satellites systematically overestimated SWH under low sea states (SWH < 0.5 m) with a mean bias of 0.17 m, which was identified as a major contributor to the overall error. This study provides a practical reference for the validation of the multi-mission SWH product from CMEMS in China’s coastal waters, and also offers a useful reference for the fusion and assimilation of multi-source satellite wave data in complex coastal environments.