The appropriate numerical modelling of a Wave Energy Converter behaviour is crucial for both the initial design phase (loadings estimations) and the productivity assessment of the real device, working in a certain installation site. The most difficult aspect to be modelled are the floater motions at resonance conditions, where motions amplitude are emphasised. Several techniques can be used, ranging from the linear Cummins' equation to fully viscous CFD simulations. This paper deals with the implementation of the 3 DOFs linear hydrodynamic model developed for the time-domain simulation of ISWEC (Inertial Sea Wave Energy Converter) device. The main advantage of the model here presented is its low computational cost. A first benchmarking between the numerical model and a series of tank testing experiments, carried out on a scaled ISWEC floater prototype, is described. Experimental results are used for the identification of the non-linearity due to the viscous effects. The novelty of the identification approach is the estimation of the non-linearity contribution along each DOF taken into account. A second comparison is done against the open source code WEC-Sim that allows the calculation of the floater instantaneous wetted surface, with the aim of obtaining a cost-benefit evaluation of the different modelling techniques.