The huge mechanical energy available in the environment, mostly in form of kinetic energy of fluids such as wind, ocean and river currents or waves, is currently harvested by cumbersome, low efficiency and high environmental impact technologies. New approaches are needed for producing more compact and distributed mechanical energy converters. Nanogenerators and related micro and nanotechnologies can help in developing new environmentally friendly and biocompatible technologies. To face this challenge, new conversion physical principle, device structures and system architectures are currently being studied and developed. This work reviews the most recent advances on nanogenerators for harvesting energy transported by liquids in the environment such as water motion in rivers and marine environments and kinetic energy in rain. It discusses the most common physical transduction mechanisms, with a focus on piezoelectric and triboelectric nanogenerators (PENG/TENG), the requirements for producing flexible devices for effective conversion and the system architectures for optimizing the fluid-device interaction for producing large and fast oscillations, such as flapping, fluttering or galloping, from quasi-static or quasi-laminar fluid motion. Additionally, the work encompasses challenges such as waterproofing and antibiofouling, important issues in sub-marine and underwater environment.