This answer gets a little complicated. There are certain frog species like the Bornean Flat-Headed that breathe only through their skin. In fact, the Bornean Flat-Headed frog doesn't even have lungs! This means that the Bornean Flat-Headed frog never needs to come up for air. However, other species of frogs do have lungs in addition to being able to breathe through their skin. These frogs will need to surface for air if the water around them runs low on oxygen. This can happen in stagnant pools of water.