The "throw 'em in and let'm swim method"
It's kindof a cliche', but I think it might actually work. You see it in movies a lot, and usually it's a dad who's unsympathetic who, as an anectode to how mean he was, taught his kids how to swim by throwing them in the deep end.
Now, I think it depends on the base knowledge you give them...in other words - don't throw your 3 year old in the deep end without telling him anything beforehand. This will end badly, with probably emotional scarring. That kid will be afraid to go in the water like he just saw "Jaws."
To rewind, we dont' have a pool. So my kids get to swim maybe 6-7 times a summer, on the infrequent visit to the pool or the aunt and uncles house. So they learn to swim slowly, in increments. My five year old, last year, just started to learn, and could maybe barely dog paddle. This year he was working on it, but was close to ending the year about where he started. He didn't want to practice, and was very afraid to go near the deep end.
Now, yesterday he tells me he wants to jump off the diving board.
Why the sudden acceleration in the schedule? I finally just threw him in.
He was saying over and over that he didn't want to go in the deep end and didn't want to practice swimming. I wanted to show him he could survive if he did go in, so I walked him over to the deep end (with him crying the whole time - I'm sure the mothers and grandmothers thought I was a total train wreck of a father (an apocalyptic dad?) - but that's okay. He protested, but I threw him in.
He sputtered and floundered, but he knew how to keep afloat and he paddled over to the side.
Praise, praise, praise! Okay, now you can play.
Next time, same thing, but farther.
Third time I made him swim to the OTHER side of the pool. And suddenly (with a little reminding) the dog paddle strokes turned into longer reaches and smiles, and now he goes down the slides into the deep end upside down and backwards.
Take that, disapproving lady who gave me an evil glance. It worked afterall.