Even if the non-contact underwater explosion phenomenon has been studied for decades and various numerical methods have been provided in open literature, its effects on naval structures, especially those made by composite materials, are, even nowadays, a matter of research. Actually, a fully coupled fluid-structure interaction model for underwater shock explosion analyses is extremely complex to set, computationally demanding and certainly not suitable for everyday design practice. In this paper, a simple finite element (FE) model including both, the structure and the fluid, accessible and easy to use in an early design phase, is proposed. Within a commercial FE software environment, an approximate fluid-structure interaction model is outlined by simplifying the fluid behaviour, in which the problem of combining computational fluid dynamics (CFD) and FE domains and equations is overcome as long as also the fluid is modelled using elements defined as finite elements. Thus, reducing the computational time and numerical efforts. As a test-case, a typical minehunter ship parallel body block is modelled, using two-dimensional multi-layered shell elements and therefore properly accounting for the characterization of composite materials. Three dimensional volumetric elements are used for the fluid instead, directly coupled to the structural elements. The results achieved applying the numerical model are compared with the structural response measured on board of a composite ship during a full-scale shock test. Satisfactory agreement was obtained allowing the validation of the model. In any case, the procedure proposed is an approximation of the real phenomenon, therefore differences in the comparison are discussed, allowing a better understanding of the phenomena. Eventually, the modelling strategy has been considered useful for the concept and preliminary design of composite structures subject to underwater explosion phenomena.