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 several studies estimates that Cilegon will experience a water crisis in 2025. In fact, at least two of the six subdistricts in Cilegon are currently already experiencing the water crisis, while four Other sub-districts are expected to experience the same conditions faster than expected. 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. So it becomes a question of how to developed a plan to stop the deficit rate of clean water then seeks to meet water needs in accordance with the sixth SDG global target. 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 founds that to be able to balance this gap, not only Cilegon had to make technical engineering efforts to increase the availability of water, but also to make a social engineering efforts by reducing their needs of water. So the balance between the need and the availability of water will achieved in 2030 in accordance with the SDGs target. This finding is expected to contribute not only to overcome and avoid the crisis of clean water in the Cilegon, but also to other coastal cities in Indonesia with similar characteristics. This study recommends that in order to achieve the sustainability of a coastal city with clean water, a systematic effort is needed by managing the balance between the needs and availability of water in the coastal city.