This paper considers the performance of a horizontal axis tidal turbine, at two sites off the Welsh coast of the UK, one in the Severn estuary and the second off Anglesey. Acoustic Doppler current profiler surveys were performed to investigate local velocity profiles. A small water plane area twin hull (Swath) bathymetric survey established the local bathymetry of the area. Energy extraction from a three-bladed 10m diameter turbine was studied using a quasi-static computational fluid dynamics model and velocity profile data from the acoustic Doppler current profiler surveys. The acoustic Doppler current profiler data were rescaled allowing further insight to the attenuation of power density through the water column at a higher tidal velocity. The velocity profile has a significant effect on power attenuation reducing the extracted power to 30-40% of peak power. The turbine performance calculation is highly dependent on the velocity used; the velocity should be monitored around five turbine diameters upstream, at the depth of its axis of rotation. The axial load on the turbine peaks at the freewheeling velocity dropping to 88% at peak power. It is suggested that the velocity at the Severn estuary site is, unlike the Anglesey site, unlikely to provide a suitable level of power to be viable.