In this study, a GIS-based system (MWQ-FES) is developed for marine water quality assessment around offshore oil platforms. The developed method consists of a fuzzy risk assessment model, a eutrophication assessment module, a heavy metal assessment module, a dynamic database, the ArcGIS Engine, and a graphical user interface (GUI). The developed GIS-based GUI system integrates the fuzzy risk calculation, eutrophication risk assessment and heavy metal risk evaluation and both spatially and visually presents the results in the form of contour maps and color-coded maps that indicate the risk levels. The assessment modules analyze a large amount of data with both spatial and temporal distributions; these data are managed by the developed system. An application of the developed MWQ-FES to a real case study in China is presented in this study. The MWQ-FES produces risk maps that depict the spatial distribution of the integrated water quality index values, the eutrophication risk level and the heavy metal risk level in the study area. The primary factors that affect the water quality are subsequently examined using the visualized results. The results of the fuzzy risk assessment model show that the general water quality status in the study area was good in Oct. 2005, May 2006, and Sept. 2007, while fair water quality occurred in May 2007. For Oct. 2005, May 2006, and Sept. 2007, the eutrophication risk levels were oligotrophic, slightly eutrophic and mesotrophic, respectively. However, the eutrophication risk level at most of the sites in May 2007 was highly eutrophic. These findings agreed with previously reported water quality variations in the study area. The heavy metal risk level in the study area exhibited a slight risk during all four investigations. As for the ecological risk in the mariculture zone near the study area, the heavy metal risk exhibited a slight risk; the eutrophication risk level ranged from eutrophic to slightly mesotrophic. A comparison with previous environmental assessment results for the same study area confirms that the developed MWQ-FES can provide a better understanding of the distribution of the water quality status and ecological risk levels. Moreover, MWQ-FES can be a useful decision-support tool for marine water quality management.