Water quality-induced water shortage is emerging as one of the main threats for the growth of the world's population and economic development, especially for coastal cities in developing nations. This paper discusses how to supply enough sufficiently clean water to such cities using the technologies of coastal reservoirs and wetland pre-treatments, as well as employing the SPP strategy. The so-called coastal reservoir is defined as a freshwater reservoir situated in seawater which sources its water from river runoff; to improve its water quality, a wetland is used to pre-treat the runoff that is potentially polluted by domestic, agricultural and industrial contaminants. Generally, the existing lakes in the urban-rural fringe are severely polluted; the Separation, Protection and Prevention (SPP) strategy can quickly restore the lakes' water quality to a drinkable standard. In this study, we take Shanghai, the largest city in China, as an example to investigate the feasibility of the proposed strategies. This investigation shows that using the technology of coastal reservoirs in Shanghai, namely the Qingcaosha Reservoir, freshwater can be pumped from the sea without requiring the desalination process and if an agricultural wetland on Chongming Island is used to pre-treat the river water prior to its storage, its water quality will be improved to Class II. It is also found that Dianshan Lake, which has been polluted and had a water quality of worse than class 5 in 2015, can have its water quality improved to class II in a short period if the SPP strategy is applied. It is expected that it will be a cost-effective and sustainable water supply method for coastal cities.