All aspects of human life require some source of energy. Fossil fuels are predicted to be exhausted in some decades. Moreover, their consumption is known to be very harmful to all living species. Wind and wave powers, as renewable energy sources with no emissions, represent better alternatives for electricity generation. The aim of the present study is to develop a model to assess a wave–wind hybrid energy system. The idea of adding a wave energy harvester to a floating or a fixed-support wind turbine structure contributes to increase the system reliability. Thus, a mathematical model was used to assess not only the components of the wind–wave hybrid system (e.g., wind turbine, wave converter, and foundation), but also the system costs and the cost recovery period. The wave energy converter selected for the hybrid device was a Wavestar prototype combined with a wind turbine. The engineering and economic improvements of the system and associated costs were calculated. Two case studies of the proposed hybrid system in the eastern Mediterranean Sea and the North Sea were assessed. It was concluded that the annual energy production from the wind–wave hybrid device in the North Sea was 64.3% higher than that in the Mediterranean Sea. As a result, the cost of energy (in USD/kWh) in the North Sea was lower than that in the Mediterranean Sea, making the hybrid technology commercially feasible.