In this study is presented a systematical investigation on the feasibility of different extremum-seeking (ES) controls to improve the conversion efficiency of wave energy converters (WECs). The implemented schemes are model-free and adopt a continuous-time approach and are based on the sliding mode, relay, least-squares gradient, self-driving, and perturbation-based methods, and are used to improve the mean extracted power of a heaving point absorber subject to regular and irregular waves. This objective is achieved by optimizing the resistive and reactive coefficients of the power take-off (PTO) mechanism using the ES approach. The optimization results are verified against analytical solutions and the extremum of reference-to-output maps.