This work provides a novel approach that combines computational fluid dynamics (CFD) with computational solid mechanics (CSM) to dynamically simulate the fully coupled hydroelastic interaction between nonlinear ocean waves and a submerged-horizontal-plate breakwater (SHPB). Based on a systematic series of simulations, it is shown that the wave damping performance of an SHPB can be evidently improved by an appropriate extent of deformation, which can be achieved through the design of its bending stiffness by varying elasticity and/or aspect ratio. The wave attenuation effect is found to be maximized when an SHPB has a deformation amplitude close to the incident wave amplitude. By accounting for the hydroelastic effect through the fully coupled CFD+CSM approach, this work presents a new computational technique that supports the design of deformable offshore and coastal structures.