Like all mammals, manatees breathe air. As a marine mammal, they must rise to the surface to take a breath. While resting this happens every 20 minutes, but when they are active, they need to take a breath every 3 to 5 minutes. To do this, they exhale very hard when their nose breaks the surface, much like a whale or a person snorkeling will do. Breathing like this means there is more fresh oxygen in their lungs, and allows the animal to stay underwater longer between breaths.
Manatees also have two other special breathing adaptations.
They can only breathe through their nose, and their nostrils have special flaps that close tightly when they dive. A manatee's mouth and teeth are specially adapted to an herbivorous life. Like elephants, they have 4 sets of 6 to 8 molars. As the front molar wears down from chewing on fibrous plants and sand, it is shed. Without hands, it can be hard to get food to your mouth.