Many people will tell that you most children under the age of 5 can't handle formal swimming instruction. Horse hockey. They can.
And considering the
number one cause of death for children 4 years old and under is drowning, every kid over the age of 6 months needs to learn how
not to drown.
Go here, watch the video.
Infant Swimming Resource | Home
Watch this one, too.
Home
They teach infants how to "right" themselves onto their back when the fall into water. That's important because water attracts kids like crazy. These kids learn progressively, starting in diapers, then until they can do it with ease while fully clothed, then as they get a little older they learn how to actually swim (1 year old and older). But initially, the important thing, is Drowning Prevention, as Cheri notes, and these folks are the real deal.
I have a friend who was bathing her 8 month old son when she had a seizure (her first one ever, and hasn't had any since) and passed out onto the bathroom floor for somewhere between 15 and 30 minutes. Her son had been through the at least the first stages of the program, and when she regained consciousness she found him happily floating on his back in the tub, eyes open, breathing, smiling. The kid's 3 years old now and swims circles around 12 year olds.
As soon as I (and my two younger brothers) started crawling my parents started us in swimming lessons. To this day I am just as comfortable in the water as I am out of it. Granted, I didn't learn under the same program as the Infant Swimming Resource program, but I do have pictures of me at 8 months old floating on my back in a pool, grinnin' like a little idiot, so whatever the program was, it worked.