This article presents a hardware-in-the-loop assessment of a moment-based optimal controller including a robust super-twisting tracking term. The aim of the overall control structure is to maximise the energy extracted by a flap-type wave energy converter. An experimental assessment is performed via hardware-in-the-loop testing, which allows experiments in a controlled environment and repeatable conditions. Key aspects of the control structure are analysed in the results, with particular focus on the behaviour of the second-order sliding mode tracking term. Robustness against modelling uncertainties and finitetime convergence are evaluated, initially for a single sea state, and then across three different sea conditions. Excellent tracking performance is observed for all considered operating conditions. Finally, the super-twisting algorithm in the tracking loop is compared with the performance of a linear controller synthesised based on Youla-Kuˇcera parametrisation, observing that the former presents better tracking capabilities, also outperforming in terms of energy extraction.