Abstract: Hovercraft are potentially vulnerable to underwater explosion threats, while the mechanisms by which the pressurized air cushion beneath the hull affects explosion-bubble evolution and blast-load transmission remain insufficiently understood. In this study, a plate structure incorporating a pressurized air cushion is established as a simplified representation of the bottom structure of a hovercraft. Numerical simulations of underwater explosions under free-field conditions and beneath air-cushioned structures with different initial pressures are performed using the coupled Eulerian–Lagrangian (CEL) method. Particular attention is devoted to the effects of the initial air-cushion pressure on bubble expansion, contraction, migration, and collapse, as well as on the propagation and transmission of shock-wave and bubble-pulsation loads. The numerical results demonstrate that the pressurized air cushion significantly suppresses the dynamic evolution of the underwater explosion bubble. Compared with the free-field condition, bubble deformation, migration, expansion, and contraction are strongly constrained, and the overall pulsation characteristics are substantially altered. Nevertheless, although the air cushion restricts bubble motion, it intensifies both the peak shock-wave load and the subsequent bubble-pulsation load acting on the structure, thereby increasing the global structural response and potential for structural damage. These findings indicate that a pressurized air cushion does not necessarily mitigate underwater explosion loads; instead, its compressibility and pressure-dependent response may enhance load transmission under certain conditions. Furthermore, bubble evolution and load-transmission characteristics under free structural boundary conditions are investigated, and the influence of structural motion on the coupled interaction among the explosion bubble, surrounding water, pressurized gas, and structure is discussed. The results provide further insight into the fluid–gas–structure interaction mechanisms associated with underwater explosions beneath air-cushioned structures. This study provides a theoretical and numerical basis for evaluating the blast and impact resistance of hovercraft and offers technical support for the design and optimization of protective structures subjected to underwater explosion loading.