If tidal-stream energy is to make a significant contribution to global renewable energy targets, understanding the influence of waves is essential as the majority of potential sites are exposed to waves. Wind waves add additional mass and momentum to the tidal flow, influencing the available resource as well as device performance and resilience. Here, the effect of waves to the resource and likely oceanographic conditions is presented. Oceanographic data from UK tidal-stream energy sites (directional wave-rider buoys, 4 and 5-beam ADCPs) were combined with dynamically coupled wave-tide regional model data (COAWST, which couples the SWAN wave model with the ROMS ocean model). The presence of waves altered the 15-minute averaged velocity profile and typically increased velocity shear. Averaged throughout a tidal cycle, the available tidal resource was also found to be reduced by ∼10% per metre wave height increase. Further, waves were observed to be frequently aligned at an oblique angle to the tidal current. Therefore, realistic oceanographic conditions, such as the interaction of waves and tides, are needed to better understand the resource and improve device design criteria; for example, wave-current misalignment should be considered in device-scale studies of performance and resilience (e.g. CFD models).