Tsunamis are one of the most destructive naturally occurring phenomena that significantly affect shoreline morphology, often resulting in deep-seated erosional features and scarps. This study investigates the seabed response of sediments in the coastal nearshore, with particular focus on the influence of subsurface hydrodynamics and air entrainment in the intertidal zone that lead to sediment instability and momentary liquefaction. Entrainment of air bubbles due to tidal fluctuations increases the bulk compressibility of pore fluid and dampens the mechanical development of excess pore water pressure due to total stresses imparted by the weight of the wave. This pore pressure response differs from fully-saturated sediments overlain by intertidal sands at shallower depths and instigates seepage at the interface of these two layers and at the seabed surface. A fully-coupled flow deformation finite element analysis was performed in PLAXIS 2D to evaluate the pore water pressure response and seepage through a saturated sand layer overlain by a quasi-saturated (intertidal) sand layer during a hypothetical tsunami event. Results of the fully-coupled flow-deformation analysis illustrates the temporal and spatial development of excess pore water pressures and hydraulic gradients that arise at depth throughout the event. Computed results illustrate that the presence of a shallow quasi-saturated layer may lead to greater instability than a continuous layer of fully-saturated sediments.