We just got done doing a swimming class for our 2 year old and this was addressed in the class. I highly recommend you take a class with your children. Classes for younger than 3 years old are done with parents in the water. It'll make both your children and you much more comfortable and safe.
http://www.nhs.uk/conditions/Sudden-infant-death-syndrome/Pa...
I'm speaking a little in hyperbole here, but you get the point I'm sure.
My mom was a great parent and wonderful overall, but that was a particularly excellent decision of hers. She was still careful with my little brother and me around water - we were enrolled in Red Cross swim classes during the summers in elementary school, and always had lifejackets on at the lake. Despite being terrible at pretty much all sports (unlike my athletically-gifted brother), I was always a decent swimmer.
When my wife and I were on our honeymoon we watched a couple throw their 6 month old (10 month maybe? wasn't walking, only crawling) in the pool over and over again. Most of the pool looked on in horror, but they had taught her the "leg kick to turn over onto her back and float" skill. Definitely gave us perspective.