Coastal structures are crucial to protect the coastline from destructive impacts induced by sea wave attacks. Structure failures, erosion, and overtopping water should be prevented or minimized during extreme events for maximizing the efficiency of the structures. In this work, we numerically investigated various shapes of seawalls in terms of force exerted on structures and overtopping water under a variety of sea waves by using an open source code based on Smoothed-Particle Hydrodynamics (SPH) method named DualSPHysics. The numerical simulations of wave forces exerted on some typical seawalls were conducted and good agreements were found between numerical and reference results. The maximum horizontal calculated forces were slightly smaller than those obtained by analytical solutions, but the differences were insignificant (less than 6%). In addition, overtopping simulations were also conducted to determine the most optimal shape of seawalls for reducing overtopping water under the same wave conditions. The results indicated that the overtopping water was reduced most significantly in case of a stepped-face wall, followed by a curved wall, an inclined wall, and a vertical wall.