Two or more marine structures deployed side by side may encounter strong water-body resonances within narrow gaps between them. It may cause significant wave loads on structures and the green water phenomenon on the deck. In this article, the transient fluid motion within a narrow gap formed by two fixed boxes suffered from incident focused wave groups is investigated using a two-dimensional viscous flow numerical wave flume. The focused wave groups adopted have the spectral peak frequency equal to half the fluid resonant frequency inside the gap. The wave fields both inside the gap and around the two-box system, the response/damping time of the transient wave surfaces inside the gap, the maximum wave forces and the ratios of the 2nd-order to the corresponding 1st-order wave surfaces/forces are investigated. It is revealed that the most dangerous place to green water is always the front edge of the two-box system. The damping time of the 2nd-order wave surface is significantly larger than that of the 1st-order one. As the incident wave amplitude rises, the ratios of the 2nd-order to the first-order wave surfaces/forces becomes increases gradually and can even exceed 100% for the wave surface, the horizontal wave force and the moment.