<p>This study examines the coupling analysis between box roll motion response and free surface oscillation in a narrow gap, utilizing a two-box system comprising a small roll box and a large fixed box. The potential flow model reveals a two-peak variation in both roll motion response and free surface oscillation across incident wave frequencies. Free decay tests indicate that these frequencies correspond to the first and second resonant frequencies of the roll-fixed two-box system. Viscous fluid flow model simulations demonstrate a two-peak behavior in roll motion response, while free surface oscillation exhibits a single peak near the second resonant frequency. Repositioning the small roll box from upstream to downstream results in increased roll motion amplitude around the first resonant frequency. The roll-box with round edge profiles exhibits beating behavior in motion response, resulting in increased roll motion amplitude across a broad frequency range. Notably, wave energy at the first resonant frequency component remains undamped by round edge profiles.</p>