I recently got back into swimming, and I was reminded of how hard it is at first. I was pretty fit, but I couldn't go for more than five minutes without needing a break. The problem was I wasn't getting enough air and felt like I was drowning.
This feeling is called going into oxygen debt, which means you are using more air than you are taking in. It is incredibly uncomfortable, and in my case causes a sense of panic. In order to keep swimming, I needed to find a way to either take in more air, use less air, or both.
Breathing while swimming is different then breathing while running or cycling (or really any other sport) because you can only breathe at discrete times. For the freestyle stroke, you can breathe at most every time an arm pulls through the water. So if you are already breathing that frequently, you can't breathe any faster. Another obstacle is speed; some speeds are more efficient than others, so swimming more slowly might actually require MORE oxygen then swimming quickly. I'm very dense, so the slower I swim the futher my legs sink into the water. This shape is much less hydrodynamic, which makes swimming harder.
So what can you do? The key for me is taking longer, deeper breaths, and focusing on my form. First, lets talk about breath. You can only breathe in when your head is turned all the way to one side, but you can breathe out any time. So breathe out slowly while you are turning your head, then once you get your face our of the water you can use that entire time for breathing in. Try to hold your head above water as long as your stroke will allow, and breathe in deeply and slowly.
Next, there is form. Of all of the endurance sports, swimming is the one where form matters the most. This is because the shape you make determines how easily you can move through the water. If your form starts to slacken, you will change your drag coefficient, which will make swimming harder. So actively thinking about your form will help you to stay in that nice hydrodynamic shape. Also, keep the tempo up; slowing down may seem easier, but because your breathing rate is tied to your cadence, you wont actually be improving your air/work ratio.
The next time I swam, I found that as I worked on these two things, then I was able to swim farther. Oxygen debt still eventually overtook me, but that is because my swimming fitness is down. As my cardiovascular system improves, I will be able to take in more oxygen with each breath. And as my swimming muscles get stronger, they will be able to do their work more efficiently, requiring less oxygen. The key is to gradually swim farther and farther each time, and eventually supply will catch up with demand and swimming will get easy.