Marine water quality can be good or bad depending on the presence or absence of different components. The delicate underwater ecosystem risks collapsing when the chemical run-off contaminates the ocean. Increased levels of chemicals can lead to toxic algal blooms, threatening the safety of marine life. Even minor damage to an ecosystem can have larger repercussions, as the harmonious balance becomes disturbed. Therefore, this study aimed to determine the physico-chemical parameters around Koh Rong Island in situ and understand the water conditions related to corals and marine biodiversity.  In this study, 15 sampling points were chosen across the island to collect water sampling to retrieve some physio-chemical parameters such as temperature, EC, TDS, salinity, pH, DO, ORP, and Chl-a. As a result, the water quality parameters were in the acceptable range of marine water quality according to the water quality standard for coastal water by EPA.  The temperature was around 31℃ for the dry season and 29.5℃ for the rainy season. While EC was about 53 mS/cm and TDS was 32.2 ppt for the dry season and 26.8 ppt for the rainy season. For Salinity, in the dry season, was about 32 psu, and in the rainy season, it decreased to 26.8 psu. The pH, DO, ORP, and Chl-a were 7.35, 8.07 mg/L, 257 mV, and 0.58 µg/L, respectively, in the dry season. While in the rainy season, pH was slightly increased to 7.83. The DO decreased to 7.96 mg/L, the ORP up to 294 mV, and Chl-a was 0.014 µg/L. Additionally, the Water pollution index (WPI) was also studied to clarify the condition of water around the study area. WPI of both seasons were in the good range with WPI = 0.72 and 0.71 respectively. with a good condition of water quality for marine biodiversity. It has shown that the coral damage or low number of fish and invertebrate abundance around the study areas has no sign related to the pollution of the water. However, basic marine water quality alone may not account for the complex and multifaceted causes of coral reef destruction, such as climate change, pollution, overfishing, and dynamite fishing. Therefore, further research is needed to investigate the specific factors and mechanisms that lead to coral bleaching and mortality.