In the present study, the hydrodynamic performance of cavitating and non-cavitating marine current turbines (MCTs) has been investigated by blade element momentum theory (BEMT) coupled with Reynolds-averaged Navier-Stokes (RANS) technique. Before employing the BEMT, two-dimensional (2-D) lift and drag forces of cavitating and non-cavitating sections of MCT have been computed by RANS. The lift and drag forces, including cavity shapes, depend on Reynolds number (Re), cavitation number, and angle of attack. The hydrodynamic characteristics of cavitating 2-D hydrofoils (sections) were calculated via a multiphase solver "interPhaseChangeFoam "from the open-source code OpenFOAM. Cavitating flow simulations around a hydrofoil were carried out with Shear Stress Transport (SST)kappa - omega turbulence and Schnerr-Sauer cavitation model. The grid convergence index (GCI) was applied to confirm the numerical accuracy of the simulations. Validation of the proposed BEMT model has been done with two different model MCTs under non-cavitating conditions. Then, cavitating flow over a hemispherical head-form axisymmetric body, the NACA 66, and the Clark-Y were selected to evaluate the accuracy of the numerical methods and turbulence models available in the solver. Then, cavitating National Renewable Energy Laboratory (NREL) S814 2-D hydrofoil was later simulated for various Reynolds numbers, cavitation numbers, and angles of attacks. By using cavitating NREL S814 2-D hydrofoil data, the cavitation phenomenon was included in the method of BEMT. The distribution of cavitation on the blade has been modeled with BEMT coupled with RANS. Power coefficients at a wide range of tip speed ratios, including cavitation distribution on turbine blades obtained from BEMT, have been compared with each other and experiments. A reliable agreement has been found for all cases studied here. It is shown that cavitation has a negative effect on the power output of MCT for all tip speed ratios and cases discussed in this study. In addition, under cavitating conditions, the tip speed ratio of the operation point that corresponds to the highest power coefficient shifts to the right side of the power curve.