CFD modelling of tidal turbines is described. For computational efficiency rotors are represented by rotating actuator lines and nacelles by partially-blocked cells. The computational implementation includes an arbitrary Lagrangian-Eulerian (ALE) approach to free-surface movement for wave motion. For a single turbine the model successfully reproduces towing-tank measurements of thrust and power coefficients across a range of TSR. Modelling of two turbines staggered streamwise shows that loads may be reduced or augmented, depending on whether the downstream turbine is in the wake or bypass flow of the upstream turbine. Where the downstream turbine is partially in the wake, individual blades suffer large cyclical load fluctuations. Turbine performance has been simulated under both regular and solitary waves. For long waves, time-varying thrust and power are found to be reasonably well predicted by scaling the non-wave case using the hub-height velocity (wave + current), provided that the reference is adjusted for changing TSR. For short waves this approach over-predicts fluctuating loads.