Wave energy resources are huge and widely distributed all over the world. Wave energy converters (WECs) are employed to harness this energy. In this paper, a heaving point absorber WEC with three kinds of Power Take-Off (PTO) systems is investigated. A fully nonlinear numerical model of the WEC has been developed, taking into consideration viscosity. A Computational Fluid Dynamics (CFD) analysis is performed in order to determine the quadratic viscous term coefficient. A physical model test has also been carried out and used to validate the numerical model. The response and energy capture characteristics of the WEC with various PTO systems are analyzed in both regular and irregular wave. The differences among the WECs with different PTOs are assessed. Finally, the annual wave power assessment and suggestions for optimizing the WEC in the real sea state are given.