Most children under six months of age swim
reflexively. When placed face-down in a pool of
water (only do this with your
infant if you are very
brave and willing to risk the
wrath of your spouse), babies will hold their breath and paddle and kick in a swimming motion.
Some children retain the swimming reflex for a longer period of time. Many parents, frantically following a wandering toddler who goes directly towards the pool, have been relieved to find their child happily swimming away after falling in. Keep in mind, however, that this is a relatively small percentage of children and you should still watch them like a hawk when they're around water.
There has been a proliferation recently of neonatal swimming 'classes' that take advantage of this very reflex to teach children how to swim at very young ages.