I can't swim. When I jump into a pool and try to float on my back, I sink like a rock.
floating requires blubber (if desiring to float without any additional movement), or a technique of creating upward motion with your hands.
I require the latter for I am without blubber.
As for swimming - I took to water like a fish.
And I almost drowned once. Didn't scare me of the water one bit. Made me respect it, that's all. Bottom water only like a foot or so beneath my kicking feet so I kept kicking off the bottom and in the process of screaming for mom, ingested a lot of water. Puked when I got back to the beach.
That taught me not to push my limits by swimming to deeper water when it was one of my first times swimming without my little floaties.
Adults, honestly come on now. While the idea of little inflatable things on your arms may sound ridiculous, and I don't recommend going that route - it is not hard to find somewhere with a recreational pool that is shallow and just give yourself time. Like all other basic activities, it is essentially instinctual but obviously it takes time to hone the skill in regards to muscle motion/repetition.
Not knowing how to swim is asking for death the first and only time an accident happens and you are surrounded by deep water. Most other things like learning a ride a bike aren't going to result in death as you won't ever accidentally find yourself on a bike.
Many places offer swimming lessons. And like I said, if you don't want a lesson, it just takes throwing yourself into a safe learning environment - a shallow pool. Start swimming... and if you start sinking, just stand up.