The numerical prediction of the power performance of a marine current turbine under a free surface is difficult to pursue due to its complex geometry, fluid–structural interactions and ever-changing free surface interface. In this paper, an immersed boundary method is used to couple the simulation of turbulent fluid flow with solid using a three-dimensional finite volume solver. Two free surface methods are proposed and tested for different conditions. The methods were then validated respectively by various studies and a coupled simulation was proposed for a marine current turbine operating under free surface waves. The power coefficients of a horizontal axis marine current turbine (MCT) with different rotating speeds are calculated and compared against the experimental data. It is found that the method is in general agreement with published results and provides a promising potential for more extensive study on the MCT and other applications.