Maybe you remember those old westerns where some outlaw is being chased by a posse, and in order to escape, he lies down in shallow water and breathes through a hollow reed. The posse, befuddled, rides on by.
Unfortunately, this trick doesn't actually work. A little physics keeps getting in the way.
Taking a breath is such a common occurrence that we rarely even notice we are doing it, but every time we inhale, we are redistributing the air in our immediate vicinity. Our chests are expanding, both filling up our insides as well as pushing aside the air that surrounds us.
That's easy enough to do with air, but water is a much denser medium. Pushing it aside takes some doing, as you quickly find when trying to wave your hand underwater. Also, unlike breathing above water, our cowboy is not just redistributing the medium. He is actually displacing the medium, water, in order to pull a pocket of air down into it.
As our cowboy begins to inhale, he will discover that the external pressure on his lungs exerted by the surrounding water is much greater than his muscular ability to inflate them. In essence, with every breath he has to push aside most of the water in the pond.
A swimmer with a snorkel can still breathe, but notice that snorkelers float right on the top. Were they to try sinking underneath, even a moderate amount, the same thing would happen.
The other problem is that a hollow reed is about as thin as a straw. Try breathing through a straw even above water and you'll soon be gasping.