The paper concerns closed monolithic rectangular tanks produced in one stage (one technological process without interruptions or dilatations) that function as floating platforms in inland waters. It presents an analysis of static work of the above-described tanks subject to the hydrostatic load of walls and bottom as well as the uniform load of the upper plate. Calculations were made with the use of the finite difference method in terms of energy, assuming the Poisson’s ratio ν = 0. Based on the obtained results, charts are provided that illustrate the variation of bending moments for the characteristic points of the analysed tanks as per specific workloads. The paper also provides test calculations for buoyancy, stability and the metacentric height for one type of the tank that was produced for the study. In this case, in addition to the above-mentioned loads, the calculation also took into account load temperature and ice load floe. The paper presents photographs taken when launching a pontoon prototype.