The problem with the “everyone can float” line is that while everyone can, most people don’t know the right way to do it.
Some people take that statement to mean that no matter how they position themselves in the water, they’ll float and be fine. Then they aren’t, so they panic, move their arms and legs in completely unhelpful manner, and maybe start to sink a bit more, panic, etc…
The easiest way to float, which everyone can do, is to take a big lungfull of air (if possible, if you’re already in the midst of going under that might not be possible,) and “lean over” in the water. Your head is almost entirely submerged, and your arms and feet will dangle from your body, and the top of your back is out of the water.
You’ll float like this, but obviously it’s a big disconcerting to the untrained because yes, you have to keep your head under the water and hold your breath. When you need more air, left your head up (gently move your arms back anbd forth to help, if you need to), exhale, then inhale, then go “back under.”
Floating on your back is pretty easy, too, but again, it’s a big of an odd sensation. Like the first example, your arms and especially your legs sort of hang there in the water. And with your legs, especially, that initial feeling when they start to sink might make someone start to panic if they think the rest of their body will follow, but it won’t. You legs will just sink to a point, then stop, and if you keep a lungfull of air, your torso will stay above the water, as will most of your head/face. Which is another part that might freak some people out. Yes, some water will get on your face, so again, someone might start to think they are sinking when it’s just the natural movement of their head and water.
Edit: Obviously, it if it’s a really choppy lake, or the open ocean with waves, or any kind of decent moving river, then this might not work…but in a pool, at least, floating is pretty easy if done right.