For thousands of years, data allowed societies to first understand the connection between water quality and public health and later to refine water and waste management strategies. In this commentary, we look at the past, present, and future of 'big-impact' data in the water sector, from ancient water systems to Paris' valorization of wastes, London's tribulations with cholera, and modern environmental standards for the protection of public and environmental health. We continue the journey of water data with emerging applications in understanding our water resources, including blue-green city planning, understanding water use trends in remote areas, water resource monitoring, and wastewater epidemiology. We conclude with a foray into the future of the water sector, where proactively managing water resources to ensure equitable and resilient water access can be informed by remote sensing, satellite-based monitoring, and machine learning. By understanding how water data revolutionized our societies in the past, we aim to inspire the use of big water data to open possibilities for managing urban and environmental water in our future.