Due to the lack of full-scale measurements of the flow around tidal turbine blades, it is not clear if unsteadiness in the background flow can induce dynamic stall and the significance of this on the rotor’s performance. To investigate, we develop a model that couples dynamic stall, rotational augmentation and blade element momentum theory with real flow measurements. We found that large, realistic waves lead to large fluctuations in power and thrust, mostly due to inviscid unsteady hydrodynamic effects near the blade tip. Conversely, flow fluctuations have little effect on the rotor’s mean thrust and power.