We started our son at 2 and he loved it. He would swim underwater and really enjoyed the water. Then we were not around a pool or any where to swim for a couple years and tried again at 4. That was a disaster. He hated the water for over a year because of the way the lessons were done and maybe something else. We tried again at 5 and the pool was better but the instruction was pathetic. He swims pretty well now, but that is mostly parent taught.
I would say look for two things, 1) is the child interested in swimming 2) is the teaching/ lessons available and are they actually going to teach your child to swim or just play in the water.