I know that newborns can supposely hold their breathe and swim under water, but no small child is safe with water. As far as water confidence goes a family splash session is as effective as an expensive lesson.
Dd started swimming lessons at three years and three months. After three terms of lessons she can now swim 5 metres on both her front and back. I believe it is developmental readiness that determines whether a child learns to swim early or not. There are children in dd's old swimming class who had lessons as babies and children in her new swimming class (all of whom can swim 5 metres) who have only had a couple of terms of lessons.
I feel the risks of baby swimming lessons (ie. glue ear, mum dying of boredom) outweigh the benefits. Its better to wait until the child is actually told enough to follow instructions and you can sit in the cafe while your little darling kicks around with a float.