As an emerging technology to harness the marine current energy, tidal stream turbine (TST) systems have been developed due to high predictability and energy density in tidal current resources. However, considering that various challenges such as swell disturbances, unknown disturbances, or parameter uncertainties may deteriorate the system performance, it is interesting to investigate alternative control strategies to the conventional proportional-integral (PI) controls. In this paper, the active disturbance rejection control (ADRC) approach is proposed to replace PI controllers in the conventional generator-side control scheme. In this approach, two ADRC schemes (cascaded and second-order ADRC strategies) are respectively applied and compared to achieve MPPT under current velocity and turbine torque disturbances. Performances of the proposed ADRC approaches are compared to PI and sliding mode control strategies. Energy production during swell wave disturbance is also evaluated under these control strategies. The comparisons show that the cascaded ADRC has better performance than the second-order approach. Moreover, the cascaded ADRC is tested under parameter variations to evaluate its robustness. The carried out simulation-based comparative study shows the effectiveness and advantages of the cascaded ADRC strategy over conventional PI controller in terms of fast convergence, overshoots elimination, and improved robustness under disturbances and parameter uncertainties.