Wave energy is one of the most promising technologies for the future of renewable energy. Its steady, even if not dashing, development, both in theoretical and applied solutions, will bring the field, in the mid-term, to the point of being a viable, economical and sustainable technology, competitive when compared to more mature ones. This paper deals with the ISWEC (Inertial Sea Wave Energy Converter) power take off (PTO) technology and control strategies. In particular it focuses on the control strategy used for harnessing energy during its first deployment, and presents the development of a new controller designed using optimal control theory. The approach to the problem of this work is not to seek for the theoretical optimal control strategy. In fact, during the test period of the ISWEC into the sea, limitations of a model-optimized control strategy came out, due to well known shortcomings of hydrodynamics modeling theory for wave energy devices, and the idea of an easy-to-tune in the field controller arose. This work points out in the direction of developing a practical and effective strategy for industrial application, with the main objective of increasing the power production while simplifying the control law by which the device harnesses energy.