Smoothed Particle Hydrodynamics (SPH) based simulations are implemented to study finite amplitude vibrations of a submerged cantilever beam in viscous fluids under a free surface. The cross section of a thin beam is modelled as a rectangular 2D oscillating rigid lamina, around which fluid field properties are computed. The study is carried out using non-dimensional frequency, amplitude of oscillations and depth of submergence. The total hydrodynamic force on the vibrating beam is extracted via SPH analysis, together with the contours of fluid field properties. Comparison is made between the results obtained with and without the free surface. We find that the presence of the free surface strongly influences the flow physics around the lamina, giving rise to non-harmonic velocity profiles and non-periodic force responses, coupled with phase lags and non-zero mean force during periodic oscillations.