Abstract: Water color is a fundamental parameter describing the optical properties of water bodies. It effectively reflects water environmental quality, and holds practical significance for water pollution control. This study focused on the coastal waters of Hainan Island and, based on the GEE (Google Earth Engine) platform, utilized Sentinel-2 satellite remote sensing data to quantitatively characterize water color index (Forel-Ule Index, FUI) in the coastal zone of Hainan Island from 2019 to 2024. We analyzed the spatiotemporal variation characteristics of water color conditions and evaluated the trophic status of the study area by examining the relationship between FUI and the Trophic State Index (TSI). The results show that, from 2019 to 2024, the FUI index within the 30 km coastal strip off Hainan Island exhibited significant spatial heterogeneity. Overall, the FUI index presented a distinct northeast-southwest polarization pattern, with higher values in the northeast and lower values in the southwest. Higher FUI values were observed in areas near the land margin, bays, and ports. Generally, the FUI index across the 30 km coastal zone did not show significant interannual variations; however, local changes were detected in certain regions, such as the eastern part of the study area and the estuary of the Nandu River in the north. A pronounced seasonal variability was identified: the FUI index showed a trend of increasing first and then decreasing over the course of the year. The FUI index was relatively high (reaching 10) from April to July (summer) and relatively low (ranging from 7 to 9) during the remaining months (autumn, winter and spring). From 2019 to 2024, the trophic status of the coastal waters within 30 km of Hainan Island was generally stable. 61.75% of the waters were classified as mesotrophic, 37.2% as eutrophic, and 1.05% as oligotrophic. Temporally, the waters were eutrophic from April to July (summer) and mesotrophic in the remaining months; at the annual scale, only 2023 was characterized as eutrophic, while the other years were mesotrophic. This study enriches the knowledge base for remote sensing-based dynamic monitoring of tropical coastal water pollution, both domestically and internationally, and provides a scientific basis for achieving regionally refined water pollution prevention and control, as well as for ecological environment management and conservation.