When we breathe, we breathe in oxygen from the air around us. Water also has oxygen, but our bodies cannot take in this oxygen.
This is why we cannot breathe under water like fish and other sea creatures. They have special organs, called gills, that make it possible for them to take the oxygen out of the water. - Sourves: howstuffworks.com; bigsiteofamazingfacts.com; answers.yahoo.com