I started my son in swimming lessons when he was 6 months old also. He loved it. We started out at Lifetime Fitness. This was a very casual setting with little instruction. It was a great way for our son to become comfortable in the water.
When he was 9 months old, we began lessons at Goldfish. I highly recommend Goldfish. It is a very structured setting with a lesson plan. My son loved it. He is now 19 months old and totally comfortable going under the water for up to 10 seconds. He likes going on the little waterslide. Also, he is learning to climb out of the pool. Goldfish really focuses on water safety while making the lessons fun.
So, I would recommend Goldfish but be aware of how expensive this option will be. It is $78/month. If you have this in your budget, then you will not regret it.