Offshore wind and wave energy are largely untapped renewable resources. However, the intermittency and the high cost of energy of these resources pose a few major challenges for their wide-scale developments. Although energy storage systems are considered to mitigate or reduce the energy variability to support a reliable power network, the proposed solutions have further increased the capital expenditure. This is primarily due to a lack of systematic techno-economic assessments of offshore renewable systems with energy storage. In addition, the integration of offshore wind and wave energy systems reported in previous literature showed a number of benefits, such as power smoothing and cost reduction. This paper investigates the offshore wind and wave energy intermittency and their dispatchability and proposes an equivalent energy storage system to achieve the same level of energy variability as the combined wind and wave system. This provides a thorough understanding of the power smoothing performance and firmness of energy supply in an offshore energy farm. The economic assessment of the stand-alone offshore wind system, the wind turbine with an energy storage system and the hybrid power unit system are conducted and compared via high-fidelity cost models. In addition, the sensitivities of three system configurations are investigated at multiple locations around the world, which are selected to address typical wind and sea states. The results indicate that the hybrid wind and wave power system has merits in reducing energy variability and enhancing ocean energy dispatchability while offering highly competitive cost, compared to the other two system configurations. Furthermore, the research aims to provide a guidance and support for the developers, investors and policymakers at the pre-planning stage of developing ocean renewable energy systems.