This paper investigates the local strength of reinforced-concrete slabs in a pontoon of the composite floating dock under uniform hydrostatic load. A refined approach was applied to calculate the reinforced-concrete slabs considering the difference in the mechanical characteristics of concrete exposed to stretching and compression. The length of the zone of fixation that impacts concrete compression and stretching has been determined, which is 0.22 lengths of the short side of the rectangular slab. To this end, preliminary calculations of stresses in slabs made from a non-composite homogeneous material were performed, at different sizes of thickness and ratios of the slabs’ side lengths. A finite-element model of the reinforced concrete slab was built, with its reinforcing elements in the longitudinal and transverse directions. The model accounts for differences in the mechanical characteristics, which are set separately for the compressed and stretched regions of concrete. The stressed-strained state of rectangular reinforced concrete slabs has been estimated for the case of complete immersion of the pontoon in quiet water under the influence of uniform hydrostatic pressure, without taking into consideration possible dynamic loads. When simulating the bottom slabs, the length of the larger side of the supporting contour was taken equal to the distance between the longitudinal walls, based on the structural size of the dock. The length of the short side varied multiple to the longitudinal quad, making it possible to acquire data for a wide range of side length ratios, from 3.3 to 1, most characteristic of ship structures. The compressed and stretched areas of concrete were simulated separately, with the mechanical characteristics of strength and rigidity corresponding to the materials used in the construction of floating docks. The charts of maximum stresses in concrete and slab reinforcement depending on the length of the short side of the supporting contour have been built. This has made it possible to determine the optimum width of the slab, which is equal to 3 m for the considered structure under predefined loading. The applied approach makes it possible to optimize the size of such structures in terms of weight and material consumption