Critical assessment of mega projects is emerging as a much-needed discipline in an era when, in many places, resource demands exceed environmental capacity. Using the Desalination Economic Evaluation Program developed by the International Atomic Energy Agency, we show that by 2030, China will have the capacity to produce 20.7 billion m3 of water annually, at $0.72/m3, as a coproduct of electricity generation through nuclear power, provided that the country favors desalination over water diversion. We calculate that the resulting production and supply chain can increase water resources in scarce regions by up to 228% while remaining affordable to poorest households. We then compare coal and nuclear desalination with the currently planned South-North Water Transfer mega project and show that, while the short-run cost of water diversion is lower, critical vulnerabilities and future resource demands favor nuclear desalination in coastal regions.