Our hossie physio dept runs post natal water aerobics. Mums take babies in the pool for 15 minutes and do rhymes, then hop out, rinse off under shower, change, feed, settle, then mums hop back in for some exercise. They can go from 6 weeks old.
DS & I started about 2 months. I got him a rashie to wear to keep in warm in the pool even though it was heated. We started proper swimming lessons at Stateswim when he was 7 months.
It's great, being in the water is so relaxing!