Researchers are now discovering that naturally occurring environmental nanoparticles can play a key role in important chemical characteristics and the overall quality of natural and engineered waters. The detection of nanoparticles in virtually all water domains, including the oceans, surface waters, groundwater, atmospheric water, and even treated drinking water, demonstrates a distribution near ubiquity. Moreover, aquatic nanoparticles have the ability to influence environmental and engineered water chemistry and processes in a much different way than similar materials of larger sizes. This review covers recent advances made in identifying nanoparticles within water from a variety of sources, and advances in understanding their very interesting properties and reactivity that affect the chemical characteristics and behaviour of water. In the future, this science will be important in our vital, continuing efforts in water safety, treatment, and remediation.