Over the years, water quality indices have been studied and used in many countries. However, research has focused on fresh water more than coastal water. For coastal countries, the coastal water quality index (WQI) is an important tool for coastal environmental management. This study presents the development of a WQI for coastal waters. A group of 20 scientists were asked to select representative parameters and their importance in the marine environment. The sub-index of each selected variable was developed based on coastal water quality criteria of different countries, and requirements of water quality for coral reefs and seabed grass. After analyzing the eclipsing and ambiguous effects, and the sensitivity of four aggregation methods, the weighted geometric mean function was used to integrate sub-indices of 8 parameters, including total suspended solids (0.18), oil and grease (0.18), total nitrogen or ammonia (0.12), total phosphorus or phosphate (0.12), chlorophyll-a (0.11), chemical oxygen demand (0.11), total coliforms or fecal coliforms (0.09), and saturated oxygen percentage (0.08). The weights of the parameter are given in parentheses. The normal weighted geometric mean function has been modified so that it could be applied in cases of lacking data. The application of the developed index for the coastal zone of Vietnam showed that it accurately reflected the state of water quality in 2017.