This paper aims to provide an indigenous knowledge system on the weather and hydrology system of the fishermen in the Southwest coastal area which is based on the research data in two specific communities: An Thuy commune (Ba Tri district, Ben Tre province) and Song Doc town (Tran Van Thoi district, Ca Mau province). The research results show that this knowledge is rich, diverse, and has been accumulated by the community for generations, especially to enable them to forecast the weather to preserve assets and lives during fish processing activities. The knowledge of the community related to weather and hydrology includes the insight of monsoon, water and storms. Offshore fishermen mastering each of this knowledge items can predict the weather to avoid the risks of sudden storms by choosing an optimal or preventive plan. This study is based on the data collected from qualitative research methods, including ethnographic fieldwork, in-depth interviews with experienced fishermen together with field notes in two areas of An Thuy commune, and Song Doc town. Besides, the paper recommends a proper proposal to preserve this knowledge in current conditions. The research results of this paper have shown the differences in local knowledge of those communities due to the weather and hydrological characteristics of the two Southeast and Southwest regions of the East Sea.