Given the advantages of flexible wave energy converters (FlexWECs), such as multiple degrees of freedom and fatigue-friendliness, there has been significant interest in these converters, which utilize structural deformation to extract energy from ocean waves. To simulate the FlexWEC interaction with ocean surface waves, a computational fluid-structure interaction approach is proposed. The fluid and solid governing equations are discretized using finite difference and finite element methods, respectively. The discrete immersed boundary method couples the two independent grid systems. A novel numerical technique is introduced to model the dielectric elastomer generator (DEG) as the power take-off (PTO). The wave energy capture performance is analysed for different PTO configurations and at various wave conditions. Based on the obtained results, the PTO damping coefficient and the relative wavelength range that maximizes the capture width ratio (CWR) of FlexWEC are determined. The wave field results also reveal the presence of periodic wave height enhancement and attenuation points around the WEC, providing potential site selection references when deploying multiple FlexWECs in an array.