While I am in favor of pool fences if only to keep other people's children safe, my main theory of water safety is teach your children to swim.
By the time they were 1, my kids turn turn over in the water from front to back and paddle to the side of a pool. By the time they were 2 to 2-1/2, they could dive into deep water and swim the length of a 25 yard pool on stomach, back, or underwater. Children, even babies, are natural swimmers. It just takes some effort to teach them how to be safe.
That's not to say that I, or another adult, wasn't always in the water with them or at the edge of the pool when they swam, but they had the skills they needed if something happened.