A simplified method for accounting for the effects of fluid-structure interaction (FSI) in sandwich structures subjected to dynamic underwater loads is developed. The method provides quite accurate predictions of the impulse on submerged structures for a large range of loads and core yield strengths. It is a simple model with two lumped masses, one of which is subjected to an incident wave and a rheological model to represent the core. It enables phenomena such as buckling of the components of the core to be taken into account and is simple enough to be used as a design tool. Comparisons with calculations of complete fluid-structure models show very good agreement.