As only a few tidal turbine developers have reached full-scale installation, the community lacks clear feedback on the validity of development tools to predict in-situ behaviour. From 2019 to 2021, HydroQuest tested its first 1 MW capacity bottom mounted twin vertical axis tidal turbine (VATT) OceanQuest at the Paimpol-Bréhat site, off the coast of Brittany, France. Thus, two years of operational data are available to analyse the behaviour of the full-scale VATT at sea. After describing the environmental conditions and the average performance of the machine, the wave effect on the turbine response is studied. The presence of strong waves multiplies almost by a factor 3 the fluctuation intensities of the torque, the rotational speed of the rotors and the turbine drag, without affecting significantly the mean quantities. The strong coherence between the loads and velocity spectra in the wave frequencies range indicates that the increase of fluctuation intensities is directly due to the periodic loads of surface waves.