Some people do not float. I've known several people who had a very hard time trying to tread water. People generally "know" that everyone can float until they meet a counter-example. I remember one swimming teacher who carefully explained to us that as long as we relaxed we would float. One student, six feet tall and very thin, proceded to sink under water with only his thumbs showing, which he twiddled for about twenty seconds.
One of my uncles used to win bets by sitting on the bottom of a pond for thirty seconds, completely under water. He did not float.