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For ships operating in combat situations, estimating the response of the bottom and side shells subjected to non-contact underwater blast load is of paramount importance. The damage severity in underwater explosion (UNDEX) depends not only on the shock factor but also on the type of fluid behind the structure (air or water). The response of a free-standing air-backed (AB) and water-backed (WB) plate has already been studied analytically by Liu and Young [1]. However, analysis for AB or WB conditions for fixed flexible plate structures has not been given due importance. In the present study, the equation of motion for the generalized single degree of freedom (SDOF) model for AB or WB plate, which accounts for its flexibility, is formulated from the principle of virtual work. However, since this model is constructed to have a global overview of the present problem without taking account of its inherent complexities, a detailed numerical investigation using MSC.DYTRAN solver is carried out for bare and stiffened plates. The results obtained from both the analytical model and numerical simulations clearly show significant reduction in displacement in case of WB condition compared to AB condition for equal shock factors. This study emphasizes the fact that WB condition can be used to our advantage in order to reduce damage associated with UNDEX in case of doublebottom or double-hull naval vessels.