Well since you've probably already found out that you have to move your feet and hands to tread water , not just let your body stay afloat by itself (if that's what you were suggesting) I just have one thing to add. I am a qualified lifeguard and swim instructor, and in our training we learned- and this is 100% true- that, some people just DON'T FLOAT. Because either
1)They have low body fat and are very lean, even if you have some body fat you might still be in clined to sink, it depends on the individual and,
2)It also depends on your ethnic background. Some ethnicities are more inclined to sink because of their body chemistry. Black people and First Nations people for example are more likely to sink than let's say a person who is Caucasian or Oriental, they would be more likely to float.
I hope this helps anyone that is having difficulty floating on their back or such, because since i was a kid, and even after the numerous swimming courses I've taken, it is impossible for me to relax on my back and stay afloat for more than a few seconds. So I know how frustrating it can be.
By the way I'm 18 and I'm applying to join the Army this year in hopefully, combat engineer, armoured soldier, or artillery air defence. Wish me luck. Bye ;
.