This study investigated how skin contributes to buoyancy control in the Florida manatee (Trichechus manatus latirostris), harbor porpoise (Phocoena phocoena), and bottlenose dolphin (Tursiops truncatus). Manatees are shallow divers and control their position in the water column hydrostatically. The two cetaceans are relatively deep divers that control their buoyancy hydrodynamically. Although the cetacean skin had been hypothesized to lower total body density (e. g., Dearolf et al. 2000, Nowacek et al. 2001), its buoyant force had not been calculated. The density of manatee skin, and its contribution to buoyancy, was unknown. Skin densities of 27 manatees, five harbor porpoises, and five bottlenose dolphins were measured volumetrically. Skin mass and density were used to calculate buoyant force. Harbor porpoise (952 kg/m3) and bottlenose dolphin (969 kg/m3) skins were less dense than seawater, and added 9 and 25 N of positive buoyant force, respectively, to total body buoyancy. By contrast, manatee skin (1,121 kg/m3) contributed 56 N of negative buoyant force, which equaled 70% of the negative buoyant force of their dense, pachyosteosclerotic ribs. Calculation of buoyant forces of the skeleton, skin and lungs demonstrates that the manatee is positively buoyant at the surface and negatively buoyant at depths of less than 10 m.