This study presents a novel approach for evaluating Multi-Body Floating Wave Energy Converters (MBFWEC) operating in complex sea states. To this end, a Transfer Function (TF) over a desired range of wave frequencies is developed by investigating power absorption capacities for four MBFWEC configurations with different floating shapes, diameters, and drafts. To examine the performance of MBFWEC systems in a practical case, this approach is applied to the wave data measured at the northern coast of the Oman Sea where complex sea states dominate. Hydrodynamic coefficients are obtained in the frequency domain using AQWA at first. Then, time domain analyses are carried out utilizing an open-source tool called WEC-Sim (Wave Energy Converter Simulator) and MATLAB®-Simulink®. Finally, the developed TF was multiplied by the region's Mean Wave Power Spectrum (MWPS) to determine the total extracted power for each configuration. The proposed spectral approach was compared to a direct method based on time history analysis, and the results demonstrated strong compatibility, thus ensuring the method's accuracy. Since the proposed TF is independent of sea states and its application is simple, it can be implemented efficiently in any region with complex sea states to evaluate or tune any FWEC device. Based on the results, it was found that a deeper draft of floating members enables the capturing of power from waves with larger periods, which is desirable in the study region. Additionally, the introduced method allows for the adjustment of the TF of an MBFWEC to be consistent with the MWPS to achieve the highest performance.