It's impossible to jump into a pool without learning something about swimming. At the very least, you start learning about how the water responds to your body. And this will remain true even if you don't consciously try. (There are experiments, for example, where people "learn" to twitch a muscle they can't consciously control because twitching it becomes correlated with turning off an annoying sound.)
Furthermore, it's been my experience that people who think they "can't explain something" really aren't trying. If you really try (and I'm sure people have), you can come up with pointers that greatly demystify swimming, even if they require you to be in a pool to start making sense.
I don't know what I would call the hard limits of a first-timer. If they swam at a competitive level the first time, and I had good reason to believe they'd never practiced, I'd have to admit I underestimated what you can learn without swimming. But it's not unreasonable that someone who was both a quick learner and was told of a known motion that closely approximates good swimming, could swim on the first try.
The first time I went ice skating a couple years ago, I flailed around dangerously for a bit, and was unable to exert much control over my path. I comically ran into a female friend and wound up in an unflattering position, and it was forceful/painful enough for all present to understand that I hadn't done it on purpose...
Then my roommate took me aside and explained what motion to do, with a few hand gestures. After about 45 seconds of explanation, it clicked in my mind, and I went out and was able to skate around quite gracefully in comparison to a few mi...
http://esr.ibiblio.org/?p=1192#more-1192
ADDED: Even if you disagree with ESR's take, and many will, this is the clearest definition I have seen on what qualia is. So it should present a useful starting point, even for those who strongly disagree, to argue from.