From a technical and economic standpoint, the suitability of a ship's propulsion system significantly depends on the bearings of its shaft line. This paper provides a comparison between white metal stern tube bearings lubricated with oil and polymer stern tube bearings lubricated with water. It is highlighted that, in real-world scenarios, the minimum lubricant film thickness in polymer bearings lubricated with water is considerably lower compared to white metal bearings lubricated with oil. In bearings with thin lubricant films, disturbances can easily cause a transition from hydrodynamic to mixed friction, leading to wear on the bearing surface and, consequently, negative effects on the ecosystem. Such disturbances can include an increase in load, temperature, a decrease in viscosity and speed, or changes in the geometry of the bearing sleeve. It is particularly shown that, under real load conditions in stern tube bearings lubricated with water, due to varying operating modes of the vessel, the influence of shaft deflection is greater than the minimum thickness of the lubricant film. General models of shaft lines in ship propulsion systems were used for this analysis. The calculation of shaft deflection within the stern tube bearing was performed using the initial parameter method. The results were verified using the MdSolids software package.