Moored floating breakwaters with a leeward boundary, assimilating the port quay walls are described by a large number of coupled variables. This complicates their design and requires a detailed parametrical and motion analysis to assess their hydrodynamic performance. A diffraction-radiation boundary value problem is developed. It arises from the interaction of linear waves on a moored floating breakwater with a leeward boundary described by a partial reflective sidewall. The effects of the sidewall clearance, structural parameters, mooring lines stiffness, and their angle of inclination on the transmission coefficient and the dynamic motion of the floating breakwater are considered. The transmission coefficient is strongly affected by the motion itself and the allowable length change in the mooring lines.