Yeah we started doing swimming for my daughter from around 9 months. She's nearly two now, she really enjoys it.My son was in the pool from about 3 or 4 months. He was born prem and a nurse suggested introducing him to baby swim classes to help develop his muscles. I'm not a strong swimmer, even though I had some lessons at school.
He went on to win a shit load of medals for his school and the state, was in nationals for water polo, and he is a keen surfer and scuba diver. I've always recommended young mums introduce their babies to the water asap; not to win medals but because we live in a country that has so much water. You need to safeguard your kids, and you are also allowing them to participate in so many activities with confidence.