Abstract: Objective Taking the free-flooding non-pressure hull structures of submarines and the double-bottom structures of surface ships as engineering backgrounds, this study selects a typical orthogonal grid sandwich panel structure as the research object and investigates its impact resistance characteristics under different operating environments. Method A planar shock-wave boundary loading method based on the Coupled Eulerian-Lagrangian (CEL) method is proposed. This study investigates the deformation and damage modes of grid sandwich panel structures subjected to varying intensities of impact load under three underwater states: water only outside the blast-facing plate, water outside the blast-facing plate and within the interlayer, and a fully water-filled state. Furthermore, the influence of water on the dynamic response of the structure is analyzed from the perspectives of energy distribution and impact-load transmission. Result Under impact loading, the face plates exhibit bending deformation or edge tearing, while the grids undergo compression-bending deformation, local wrinkling, or crushing failure. In terms of energy absorption, the water layer between the panels reduces the structural strain energy by 18.9%, while the back-layer water reduces it by 91.9%. In the fully water-filled state, the structural stress distribution becomes relatively uniform. Additionally, the presence of water attenuates shock-wave reflection at structural walls, allowing most of the incident energy to be transmitted into the water domain behind the plate. Conclusion The findings elucidate the differences in impact response, damage extent, and failure modes of grid sandwich panel structures under different water-medium environments, providing a basis for structural design in relevant engineering applications.