This paper studies the dynamics and power extraction of a U-Oscillating Water Column (U-OWC) device. A three-dimensional hydrodynamic model, based on the boundary integral equations method, is implemented. The U-OWC width and wave direction are analysed and compared with a two-dimensional model based on the eigen-function expansion method. Results show that as the device width increases, the power extraction per unit width tends to the two-dimensional value. For small converter widths, which are expected to be implemented in practical applications, three-dimensional effects are relevant and provide a systematic improvement in power output per unit width, due to their influence in the device hydrodynamic characteristics.