Potential (inviscid-irrotational) and Navier-Stokes equation-based (viscous) flows were numerically simulated around a 2D floating rectangular body with a moonpool; particular emphasis was placed on the piston mode through the use of finite volume method (FVM) and boundary element method (BEM) solvers. The resultant wave height and phase shift inside and outside the moonpool were compared with experimental results by Faltinsen et al. (2007) for various heaving frequencies. Hydrodynamic coefficients were compared for the viscous and potential solvers and sway and heave forces were discussed. The effects of the viscosity and vortex shedding were investigated by changing the gap size, corner shape, and viscosity. The viscous flow fields were thoroughly discussed to better understand the relevant physics and shed light on the detail flow structure at resonant frequency. Vortex shedding was found to account for the most of the damping. The viscous flow simulations agreed well with the experimental results, showing the actual role and contribution of viscosity compared to potential flow simulations.