Emergent rubble-mound breakwaters have been one of the main sea-defense structures used in Italy and the EU. Their functionality is related to the dissipation of the incident wave energy so that the transmitted component is within acceptable limits for the safety/stability of a coastal site. However, they can pose some environmental issues e.g. the impact on the seascape. A possible approach to attenuate these impacts is to convert preexisting emergent breakwaters into submerged breakwaters providing that the transmission coefficient remains the same. In this paper a simple approach for the functional design of such submerged breakwaters is proposed for the interest of designers. It was developed using a composite model based on: i) a laboratory process model; ii) the analysis of the process model results; iii) a computational phase using these results in the production of useful output data. The results of the computational phase have been summarized in graphs that can be used to obtain the design parameters for submerged breakwaters for given values of the design parameters of preexistent emergent breakwaters.