My experience with people who don't float easily is that they're not expanding their lungs enough. When you train in diving, you realize just how much different your buoyancy is when you have a full breath compared to an exhale. On my first few dives, I realized that I didn't trust the equipment enough and kept my lungs full while only taking shallow breaths. Learning to dive with mostly empty lungs made it so much easier and allowed me to get rid of all but one weight on my belt.

When I started training in freediving (apnea), I learned how to take much fuller breaths. I can almost guarantee that if you haven't done that kind of training, strengthened your diaphragm and learned 3-zone breathing, you aren't taking in nearly as much air as you could.

Our bodies are mostly water and the amount of mass that we have that's denser than water isn't that much denser. But water is a lot denser than air and the more you can add air volume, the more buoyant you become. Learning to float is a matter of learning to maximize your volume. Some density numbers from the internet...

Salt water: 1.03g/ml Fresh water: 1g/ml Muscle: 1.06g/ml Bone: 1.1g/ml (approximate...couldn't find a good source for this) Fat: 0.9g/ml Air: 0.0023g/ml

The numbers seem to back up your instructors. As you can see, all these values are fairly close except for air, so adding more air to your volume can dramatically change your buoyancy. Wikipedia lists the average adult male lung capacity at 6L, so a 5L breath should be achievable for most. In salt water, that 5L breath will add more than 5kg of buoyancy, which is enough to float around 400 lbs of muscle or 176 lbs of bone. In fresh water, that same 5L breath will add a bit less, enough to cancel out 193 lbs of muscle or 121 lbs of bone. There's no way that I can know your body composition, but those numbers are very substantial. The more likely is that you're just not getting anywhere near 5L of air in your lungs.

If you're ever into you're trying it again, look up some stretches to open up the lungs. Then practice breathing shallowly with nearly-full lungs. If you're able to do that, you'll probably be able to float laying on your back, where your head is mostly submerged...remember, every part of your body above the surface counts against your buoyancy, for the same reason that air in the lungs matters so much.

Maybe your bones are too dense, though to be honest, I've never heard of anyone with your issue in floating/treading water. I've always assumed that everyone has enough buoyancy to float most of their body.

I'm super skinny and sink straight to the bottom of a pool. Same issues :(

I float, but not much more of me is out of the water than I could cover with my hand. Even with more fat than I like to carry, I can stand on the bottom of a swimming pool. This greatly amuses my wife, who can lift her toes out of the water, while mine are pointing straight down.

My friend claimed he couldn't float . And it appeared he couldnt when he tried. But I put my full mask scuba mask on him and magically he could float. My theory is that he was craning his beck when his face got closeish to water, messing with his form

Neither do I. My legs always sink. The long muscular legs and low body fat might kill me one day.

You might have low fat, a lot of "white" muscle (explosive sprint), and not a lot of "red" one (endurance). White is heavier than red. Also, bone density and lung volume might be factors.

I have the same problem! Sinkers unite!

My theory is that I keep less air in my lungs on a normal out breath than others.