Why can’t we breathe under water? Human beings breathe in oxygen from the air through lungs. There’s oxygen in water too, but we can’t use it, as our lungs can only take oxygen from air. If we tried to take a breath underwater, our lungs would fill up with water and we wouldn’t be able to breathe. Fish can breathe under water through gills, which are specially designed to take oxygen from water. If a fish is taken out of water, its gills dry out and it cannot breathe.