When is the best time to start taking the little one into a swimming pool?

Babies are natural born swimmers and you can sign up for classes once they are 6 months old. Younger babies can opt to join Babyspa but I personally think it's a waste of money. I've signed up for a trial lesson for my son with Little Splashes during the baby fair in January. He hasn't started yet but when he does, he will be about 7 months old. I've heard good reviews about Little Splashes and they have heated indoor pool which I found was important to me. Each lesson is only 30minutes long.