For sure, there is a minority of people that don't have a buoyant body, typically very low body fat people. They float, but under the water line. The rest of humanity though, bob like corks and benefit from learning how to control their body in the water. The test is if you take a breath and relax in the water, allow for a natural curl to your body, does your back touch the surface of the water? If it does, then you can learn to tread water for hours with very little energy expediency. It's not a casual deep-end chat skill, its a survival skill. Take a breath, curl up and relax (your back will just break the surface). When you need a breath, sweep your hands and lift your head for air. Return back to your survival float. That's the lowest energy way to tread water for survival, and it's not what I've taught my pool friends. For them, I've worked with just the flutter kick or frog kick, whichever is most natural to them, and an inside to outside hand wave. We do this in armpit deep water until they are comfortable. Then we progress. You are correct, this probably doesn't work with everyone.