Abstract: Reinforced concrete (RC) slabs are the primary load-carrying member of underwater facilities. They can suffer severe local failures such as cratering, spalling, or breaching as a result of underwater close-in (UWCI) explosions. In this study, we established a fully validated high-fidelity finite element analysis approach to precisely reproduce the local failures of RC slabs after a UWCI explosion. A recently proposed dynamic constitutive model is briefly introduced to describe wet concrete. The effects of free water content on the material properties, including the tensile/compressive strength, elastic modulus, strain rate effect, failure strength surface, and equation of state, are comprehensively calibrated based on existing test data. The calibrated material parameters are then verified by a single-element test. A high-fidelity finite element analysis (FEA) approach of an RC slab subjected to a UWCI explosion is established using an arbitrary Lagrangian-Eulerian (ALE) algorithm. Simulating previous UWCI explosion tests on RC orifice targets and underwater contact explosion tests on saturated concrete slabs showed that the established FEA approach could accurately reproduce the pressure-time history in water and damage patterns, including the cracking, cratering, and spalling, of the RC orifice target and saturated concrete slab. Furthermore, parametric studies conducted by simulating an RC slab subjected to a UWCI explosion showed that: (i) the local failure of an RC slab enlarges with increased charge weight, reduced standoff distance, and reduced structural thickness; (ii) compared to a water-backed RC slab, an air-backed RC slab exhibits much more obvious local and structural failure. Lastly, to aid the anti-explosion design of relevant underwater facilities, based on over ninety simulation cases empirical formulae are summarized to predict local failure modes, i.e., no spall, spall, and breach, of water- and air-backed RC slabs subjected to a UWCI explosion.