Major transitions between marine and freshwater habitats are relatively infrequent, primarily as a result of major physiological and ecological challenges. Few species of cartilaginous fish have evolved to occupy freshwater habitats. Current thought suggests that the metabolic physiology of sharks has remained a barrier to the diversification of this taxon in freshwater ecosystems. Here we demonstrate that the physical properties of freshwater provide an additional constraint for this specious group to occupy freshwater systems. Using hydromechanical modelling, we show that occurrence in freshwater results in two- to three-fold increase in negative buoyancy for sharks and rays, which carries the energetic cost of increased drag due to lift production. The primary source of buoyancy, the lipid-rich liver, would need to increase 8-fold in volume if individuals were to maintain the same net buoyant effect that would occur in marine waters, suggesting that only limited compensation for the increased negative buoyancy is possible. The first data on body density from two species of elasmobranch occurring in freshwater (bull shark Carcharhinus leucas and largetooth sawfish Pristis pristis) support this hypothesis, showing similar liver sizes as marine forms but lower liver densities, but the greatest negative buoyancies of any elasmobranch studied to date. Our data suggests that mechanical challenges associated with buoyancy control may have hampered the invasion of freshwater habitats in elasmobranchs, highlighting an additional key factor that may govern the predisposition of marine organism to successfully establish in freshwater habitats.