One of the most commonly misunderstood things in swimming is how to manage our breath.  Far too frequently I meet swimmers who have been told by somebody (unfortunately sometimes their swim coach) that they should empty their lungs and fully refill when swimming.  
When running, cycling, walking, jogging or any other form of exercise we do not consciously empty or lungs and not should we when swimming. We want to breathe naturally relatively to effort level.
Try fully exhaling and fully inhaling when going for a walk and see how it makes you feel (dizzy probably).  Emptying your lungs will often your 'flight or fight' instincts we have and make you uncomfortable, it will also make you less buoyant.  
Breathe naturally and easily, gently out through the nose and in through the mouth.  Don't hold your breath of course but absolutely do not try and empty and fill your lungs every time you breathe when swimming.  If your coach tells you to empty your lungs when swimming, my best advice is find another coach.
​Here is an article I wrote for Outdoor Swimmer magazine on breathing.