The key here is to 'exhale' when your face is in the water. Do this before you turn your head to get more air. This is where many get it wrong. If you don't empty your lungs, you will not be able to get more air when you breath... Good swimmers learn to do this effortlessly thus enabling them to swim as long as they want...
Great Advice! However, I tell the older folks who live in our gated community to use a snorkel and swim mask if they can't get enough air. After all, I rather see them in the pool getting a good workout than staying away from the pool because they can't breath properly while swimming.