The fast-growing threat of energy crisis due to the increasing utilization of fossil energy has aroused research on sustainable and green energy resources. As the largest energy reservoir on earth, water is considered as a promising renewable energy carrier for ameliorating global energy issues. Water energy evolves into diverse forms that dominate the energy transfer occurring in various natural phenomena such as streaming, waving, evaporation, and raindrops. Apart from traditional bulky mechanical generators driven by water gravity in hydropower stations or hot steam in thermal power plants, extensive efforts have been devoted to developing new technologies that can harvest water energy in a more delicate way, which can be mainly divided into two types.