Cruising speeds of Mesozoic marine reptiles have been estimated in the past by using a mathematical model of energetic equilibrium during steady swimming. This method suffered from a significant tendency to overestimate speeds of extant cetaceans for no clear reason, which raised questions about the validity of the approach itself. The present study identifies the factors that caused this shortcoming and proposes corrections and some additional modifications. These include the use of more accurate body shape models, updated metabolic rate models, and optimal rather than critical swimming speeds. The amended method successfully approximates published optimal speeds of several extant marine vertebrates, including cetaceans, showing that the basic framework of the energetic approach is valid. With this confirmation, the method was applied to Mesozoic marine reptiles, by assuming three different metabolic rate categories known in extant swimming vertebrates (i.e., average ectothermic, raised ectothermic, and marine endothermic levels). The results support previous inferences about the relative cruising capabilities of Mesozoic marine reptiles (i.e., ichthyosaurs > plesiosaurs > mosasaurs). Stenopterygius, a thunniform ichthyosaur, was probably capable of cruising at a speed at least comparable to those reported for some extant thunniform teleosts with similar diets (∼1 m/second).