This paper aims to show that shape optimization can be efficiently applied to coastal engineering. This is a feasibility study for the design of defense structures for wave attenuation in deep water and against sandy beach erosion. Hence, we consider short wave propagation on a constant depth toward a vertical wall or sea cliff and long wave propagation in a domain with slow but arbitrary depth variations. The governing equations are either based on the Helmholtz wave propagation model or on a parabolic nonlinear refraction/diffraction model in shallow water. In each case, the shape of defense structures is modified in order to minimize a pre-defined cost function taking into account the strength (energy) of the water waves.