My feet sink if I try to float on my back w/o swimming. But there's another float called the dead man's float that works fine for me. It also works well in rough water:
Thank you for this. I don’t think we tried this technique — but yes my legs are usually the first thing to go down, pretty much instantly. And I have a shorter torso, longer legs (when I sit down with someone several inches shorter than me, we are at the same eye level)
https://www.sportsrec.com/survival-float-6582.html