This paper deals with the efficiency maximization of a wave energy converter (WEC). The WEC is an oscillating water column (OWC) device and drives a permanent magnet synchronous generator (PMSG) through a bidirectional flow impulse-turbine. The converter faces challenges such as large peak-to-average power ratio, low overall efficiency, and inefficient energy absorption for regular and irregular sea states. In this context, a higher order sliding mode controller (HOSMC) was proposed, and its gains were optimized to control through the best efficiency point tracking (BEPT) of the turbine. The flow through the turbine-passage was simulated by the computational fluid dynamics (CFD) technique, and the BEPT characteristics were obtained. An adaptive inertia-weight particle-swarm algorithm and a grouped grey-wolf algorithm were used for optimization. The Optimized HOSMC reduced chattering, minimized the reaching time and improved the mean efficiency by about 67% compared to the uncontrolled cases. In addition, the relative improvement of the mean efficiency was at least 4.8% compared to conventional controllers. The controller reduced the peak-to-average power ratio of at least 35.6% relative to the uncontrolled case of the turbine under different sea states. © 2019 Elsevier Ltd