The present study reports on a series of laboratory experiments aiming to understand and to optimize the deployment of seawall defences over low-lying coral reef flats. The study is performed in a purely cross-shore configuration, with varying wave forcing, water depth, and seawall shapes and positions along the reef flat. The observations reveal the individual and combined effects of the external parameters. A higher seawall efficiency is reached for low waves, shallow water depth, seaward position, and a stepped seawall shape. A dimensionless formulation is proposed to predict the short wave transmission coefficient dependency on the seawall freeboard, incoming wave height, berm width, and seawall shape.