There's no clear answer to this. Our local public swimming pool has swimming lessons starting at 5 months. I've heard some parents bring their babies swimming as newborns, while others may prefer to wait till they get their first round of vaccinations. It's really up to the parents I think.
Just remember to keep baby warm as they lose heat quicker than adults, so a heated pool may be more suited. Also it's good to try to condition baby to water before bringing them to a pool. Check out this website for videos:
Kids Alive - Do the Five!