This paper discusses efforts in building the sustainability of coastal cities against the water crisis, with the study area in Cilegon, Banten Province. The study results in several studies that estimate that Cilegon will experience a water crisis in 2025. This is a challenge, especially in the effort to fulfill the sixth global target of Sustainable Development Goals, namely to fulfilling 100% clean water needs by 2030. To answer the research question, this study uses a system dynamic method by observing the simulations of relations between variables to be able to balance the gap between clean water needs and the availability of clean water in Cilegon. This study found that to balance this gap, Cilegon had to make technical engineering efforts to increase water availability and make a social engineering effort by reducing their needs for water. So the balance between the need and the availability of water will be achieved in …