Analytical and numerical modelling techniques have been used extensively to predict the performance and power output of these devices using linear, inviscid and irrotational theory with the knowledge that nonlinear effects become relevant in extreme cases. This study applies Reynolds averaged Navier-Stokes (RANS) computational fluid dynamics (CFD) model to simulate the diffraction and radiation problems for a single submerged spherical WEC operating in both heave and surge. Wave and device oscillation amplitudes from 30 mm to 60 mm and frequencies from 0.8 Hz to 1.2 Hz are employed to examine the fluid dynamics near the spherical WEC as the hydrodynamics deviate away from the linear regime. Results of the hydrodynamic coefficients from wave basin experiments are used to validate linear finite element and CFD models for small wave amplitudes. The nonlinear CFD model is then extended to model larger amplitudes. The hydrodynamic coefficients are here found to be amplitude dependent with free surface interactions being a key component of the deviation from linear theory. The rate of these deviations from low wave height, linear values via increasing wave heights is also found to vary with frequency. The outcomes highlight limitations in the linear approach and address the factors most important to WEC performance.