You breathe in oxygen from the air. There is oxygen in water too, but the human body cannot take this oxygen from the water. If you tried to take a breath underwater, your lungs would quickly fill up with water and you wouldn’t be able to breathe.
Fish and other sea creatures, however, can breathe underwater. They have organs called gills which enable them to take oxygen from the water. But if a fish is taken out of water, its gills dry out and it cannot breathe.