Water is the most important substance for human’s survival. People are always chasing water for better life, this is why old civilizations were located near large rivers like Yellow River, Indus River, Grange River, Nile River etc., and all megacities always appear at deltas of large rivers. In the 21st Century, the biggest challenge is how to provide sufficient quantity of fresh water to urban communities. We believe that coastal reservoirs would dominate the future water supply rather than other alternate water source strategies like existing inland reservoirs, desalination plants, wastewater reuse and rainwater tanks etc. A coastal reservoir is a freshwater reservoir inside seawater. The statistics shows that the world discharges 40,000 km3/year of water into the sea, most of it is floodwater. Human society only uses 5.6% of the surface runoff. Thus, it is not that the world is running out of fresh water, it is that fresh water is running out of the world. To solve the water crisis, few coastal reservoirs have been constructed, their advantages and disadvantages are reviewed, problems are summarized, and possible solutions for future coastal reservoirs are suggested.