As freshwater resources are becoming increasingly scarce, unconventional sources of water should be given new consideration. In coastal cities, seawater, with minimal treatment, can be used for toilet flushing, reducing the demand for freshwater. Currently, it is practised on a large scale only in Hong Kong. This study estimates the cost of seawater flushing and compares it to the cost of wastewater recycling for 15 major coastal cities around the world: Buenos Aires, Chennai, Hong Kong, Jakarta, Karachi, Los Angeles, Miami, Mumbai, New York City, Osaka, San Francisco, Shanghai, Singapore, Sydney and Tokyo. While seawater flushing requires a separate network of mains and, therefore, a greater capital cost, wastewater recycling has a higher ongoing treatment cost. Wastewater recycling, depending on the potability of the recycled water, may also require a separate network of mains, but one with a lower maintenance cost due to its lower vulnerability to corrosion compared to seawater mains. This study finds Chennai, Mumbai and Shanghai to have strong potentials for seawater flushing. That these cities have among the highest population densities in the world and are in the developing world explains their relatively lower unit costs for seawater mains.