A finite element (FE) model is developed to study the breaking wave-induced dynamic response of the porous seabed and the rubble mound foundation under a composite caisson-type breakwater. The breaking wave impact pressure distributions on the front face of the breakwater are calculated using a recently proposed method. In this study the focus is on the dynamic response of the foundation materials underneath the breakwater. The impact response of the seabed and the rubble mound is presented in terms of pore pressure and shear stress induced around the breakwater. A complete formulation of the fully dynamic response requires inclusion of the inertial terms associated with both the motion of solid skeleton and that of pore fluid. However, partly dynamic and quasi-static idealizations are also possible. The objective of this study is to investigate the effects of the inertial terms on the breaking wave induced impact response of the seabed as well as the rubble. The effect of seabed saturation on the response from different formulations is also examined.