Why do babies seem to know how to swim well?

Yes there are natural reflexes a young baby has which can help them learn to swim. But you don’t have to force the situation to take advantage of them. You need a well trained teacher, because swallowing too much water can be dangerous. But done safely, it’s awesome. You are catching them before they learn fear- and by your keeping them safe, they will continue to want to learn.
Two important things to note.
One is that you can’t do this just for a summer if you want it to have any effect on future swimming skills. If you stop taking your 6 month old in September, he surely will have no conscious recall of the experience next June when he’s 15months old. I went year round with mine, at an indoor pool of course, up to about age 3.
And two, an infant’s head is too large in proportion to their body for them to be able to lift it out of the water for a breath for some time. All of my 4 kids could do this and swim independently between age 2 and 3, and one actually did it at just 19 months, something his experienced swim teacher had never seen before. There are back float methods that teach them to roll on their backs to breathe. It’s supposed to make your kids ‘watersafe’ if they got into a body of water without someone seeing them. I don’t like them. I believe it’s the job of adults to keep little ones safe. I don’t like passing that responsibility on to a baby or very young child, certainly not in a way that causes fear. And it’s common for this type of thing- or even the lessons I did but less so simply because we know they can’t lift their heads the first 2–3 years- to create a false sense of security that makes adults less vigilant when water is near. I don’t care what method you use or how well your child does, always be hyper vigilant with small children around water!