The Resonant Wave Energy Converter 3 (REWEC3) is a wave energy converter belonging to the family of Oscillating Water Columns (OWCs). It comprises an oscillating water column and an air pocket connected to a turbine, as in the traditional OWCs. In addition, it has a small vertical U-shaped duct connecting the water column to the open wave field. Because of this particular geometrical configuration, it is also known as U-Oscillating Water Column (U-OWC). The first full-scale prototype was constructed in the Port of Civitavecchia (Rome, Italy), in the context of a major port enlargement. This paper shows some results of the monitoring activity pursued on one U-OWC chamber equipped with sensors measuring water and air pressures. The activity has been conducted for about 1 year. The energetic performance of the plant is investigated in the paper, via the estimation of the capture width ratio.