I get this question almost everyday from new swimmers, "Why do I get out of breath when I swim freestyle? What's wrong with my breathing technique?"
The answer is that it has NOTHING to do with your breathing! Instead:
1. Technique. You need to look at your swim technique. Often the problem is a poor catch at the front. When swimmers do not feel the propulsion from their stroke, they compensate by kicking harder. This in turn consumes a lot of oxygen as the quads and hamstring are the largest muscle group in the body. So work on your catch instead. In an efficient stroke, 70-85% of your forward drive comes from your catch. The leg kick is more to provide balance and avoid your legs from sinking.
2. Pacing. A more common problem in those who can swim, is poor pacing. They swim at their fastest speed and aim to hold that over long distances but end up only getting to the 50m mark. Knowing they can swim that fast, their ego gets in the way of dropping their speed to a 'sustainable speed'. Finding your maximum sustainable speed (other common terms are critical swim speed 'CSS' and threshold speed) is what will help you swim at your fastest without getting out of breath.
3. Training. You may be a strong athlete in your run and bike disciplines. However, as a new swimmer, you need consistent training to develop and condition your muscles for swimming. This is especially so as we do not swim as often as we walk or run in our daily lives, for example. Over time, you will develop muscle endurance for swimming. Our muscles must be trained in a way that is very specific to the sport, be it swim, run or bike; muscle fitness does not cross-sports.
So remember, if you feel out of breath, work on these in sequence:
1) technique 2) pacing 3) training.