It sounds weird, but yes, you have to breathe out underwater as many times as you take your breathe in. Here is why: our body creates CO2 as a byproduct of exercising. When too much CO2 build up in your body, you get those urges to take a breathe because your brain things you don't have enough O2. So, when you breathe out underwater, you get rid of that CO2 that builded up in your body. Remember to inhale above the water with your mouth, you can exhale underwater with both your nose and your mouth.