On land, breathing is quite natural, even during exercise. But in water, it's evident that humans are hardly fish-like.
Holding the Breath
A very common breathing error seen in athletes is holding the breath. This generally occurs while the swimmer's face is in the water, during what should be the exhalation. Because it may feel unnatural to breathe into water, instinctively, the athlete then holds his or her breath and inhales/exhales back-to-back quickly while the face is out of the water.
It's important to understand that when a swimmer feels desperate for air, it's actually from a buildup of carbon dioxide from holding the breath, not a lack of oxygen. Holding your breath while swimming creates an irregular breathing pattern that is unable to deliver appropriate amounts of oxygen to the muscles and organs.
Beyond that, when the lungs are full of air, they are incredibly buoyant like balloons, causing the chest to pop up too high and the hips and legs to drop. This creates a massive amount of drag that can easily be avoided by employing a consistent breathing pattern.
The rise and fall of your breath in the water should be nearly identical to when you regulate your breath on land. Focus on smoothly exhaling half of your lung capacity through your nose and mouth, as if you're sighing, the entire time your face is in the water. Next, aim to take a small, relaxed sip of air as your head and body roll to the side on each stroke cycle.
This breathing pattern of ventilating the lung every stroke cycle (every two strokes) allows for a steady supply of oxygen, as well as a consistent removal of the by-product, carbon dioxide.
Not only will this allow for a rhythmic breath every two seconds or so, but it will also help to remove tension in the body caused by holding your breath.
Inhale, exhale, inhale, exhale: Let the breath be your tempo that syncs your stroke.