Wave-to-Wire numerical models being developed for the study of wave energy converters usually make use of linear potential flow theory [[1], [2], [3], [4], [5]] to describe wave-structure interaction. This theory is highly efficient from a computational perspective. However, it relies on assumptions of small wave steepness and small amplitude of motion around mean positions. Often, maximization of wave energy converters’ energy performance implies large amplitude motion [[6], [7], [8]], thus contradicting the assumption of small amplitude motion. An alternative approach is to linearize the free surface conditions on the instantaneous incident wave elevation (Weak-Scatterer approach [9]) while the body conditions are evaluated at the exact body position. Studies of wave energy converters’ dynamic response using this method are expected to be more accurate, while maintaining a reasonable computational time. With this aim, a Weak-Scatterer code (CN_WSC) was developed and used to study two submerged wave energy converters. The first is a heaving submerged sphere and the second is a bottom-hinged fully submerged oscillating flap. They are inspired respectively by the Ceto [10] and WaveRoller [11] devices. Initial calculations were performed in linear conditions first to verify the CN_WSC against linear theory. Subsequently, calculations in nonlinear conditions were performed, using large wave steepness and amplitude of body motion. In linear conditions, results of CN_WSC showed good agreement with linear theory, whereas significant deviations from linear theory were observed in nonlinear conditions. As amplitude of body motion increases, linear theory tends to overestimate energy performance in comparison with Weak-Scatterer theory. In contrast, with smaller amplitude of motion but larger wave steepness, the opposite result is obtained: energy performance is underestimated by linear theory compared to Weak-Scatterer theory.