This paper describes a campaign of WEC (wave energy converter) testing and presents a selection of the results related to the measured motions and mooring tensions. A 1:20 physical model has been successfully deployed using a three point mooring installed at sea (Strangford Lough, NI) in 10 m depth. In calm weather the overall dynamics of mooring tensions is dominated by the tidal cycle due to the progressive lifting of the heavy chain with the increase in water depth on the flood and gradual lowering on the ebb. In fresh winds the dynamics is very complex, but can be studied with the aid of mathematical modelling. A simulation model was used to assess the dynamics of the mooring lines, and the results of open water tests have been compared with the model’s performance. The results indicate that, in general, the model shows a reasonable agreement with the observations. The WEC’s motions and the measured loads on the leading mooring line appear to relate to the concurrent environmental conditions. The data obtained can therefore be used for the model’s calibration and further improvements, which is valuable for improving the WEC’s design and operational characteristics. This may be important not only in relation to the issues of reliability and power take off, but also in terms of minimising the adverse effects of mooring lines on bottom sediments, as well as indirect effects of the eroded particles on a wide range of aquatic processes.