This volume introduces particle image velocimetry (PIV), a technique for water wave measurement in the laboratory and in the open ocean. It discusses the turbulent dissipation, Reynolds stresses and vortical structures in boundary layers of the sea bed, as well as ships, ship wakes, propulsion hydrodynamics, cavitation and free surface waves. Upwelling behind crests of micro-breaking ocean surface waves (important for the exchange of greenhouse gases between air and water) and large amplitude internal solitons in the ocean are measured. The book includes velocities and accelerations in breaking ocean waves, run-up, interaction between strong waves and breakwaters, as well as a concise description of the state-of-the-art PIV technique.