Most reptiles breathe as humans do: They inhale oxygen through their nostrils or mouths. However, aquatic turtles and some snakes also can extract small amounts of oxygen from water. While snakes usually surface to replenish their oxygen stores, they can absorb some oxygen from the water via their skin. Turtles pull oxygen from the water via the skin on their necks and cloacas -- the chamber through which feces, urates and eggs pass.
Most reptiles are ectothermic animals, meaning that their biological processes occur faster when the animals are warm and slower when the animals are cool. This means that the higher the temperature, the more oxygen reptiles require. Therefore, at cold temperatures, such as during winter hibernation, some snakes and turtles can extract enough oxygen from the water to meet their needs without needing to surface for air.