Edge supported metallic sandwich panels subject to underwater blast respond in a manner dependent on the relative timescales for water cavitation, core crushing, and structural response in bending. This article examines the response to impulses representative of an (especially relevant) domain: wherein the water cavitates before the core crushes. In some cases, core crushing ceases before the overall structural deformation is complete, leading to what is termed strong-core response. In other situations, core crushing continues beyond the stage at which the dry-face of the sandwich panel ceases to deform, a behavior that is known as weak-core response. It is known that weak-core response provides superior panel performance in terms of three performance metrics: the back-face deflection, the tearing susceptibility of the faces and the loads transmitted to the supports. The relevant regime of behavior is determined by the magnitude of the transmitted impulse from the water, the relative strength of the core, and the aspect ratio of the panel, so that a panel must be designed carefully against a known threat to achieve optimal performance. This paper initiates the development of a map of response for sandwich panels having I-core geometry, delineating strong, weak and other behavior in terms of some of the design parameters mentioned above. The effect of fluid – structure interaction is included in the simulations, and these results are contrasted with those in which fluid – structure interaction is omitted and an initial impulse is instead given to the wet-face of the panel.