I agree with those who say don't force it.
DS went at the age of 4 months. Then I went back to work so it stopped at about 1.5 years. Then I took him back at just turned 3.
He was fine on the first lesson...until he fell underwater. After that, he just wanted to do his own thing, wander aorund the pool, climb in and out when he wanted.
As the lessons were structured, I was spending each half hour, trying to get him to come in and do what the other kids were doing. Anyway, after 4 lessons, we stopped going and he was not bothered on the slightest. He is starting a 'freestyle' weekly session with his dad in Spring.
As long as baby is confident in the water then being able to 'swim' as such is no big deal yet (says she who can't swim very well at all )