S.F.
We take "Mommy and Me" swim lessons at the Jewish Community Center - you don't have to be a member to do the lessons there. M&M classes are for 6 months - 3 years and are about getting the child comfortable in the water, teaching the parent to handle them safely in the water, etc. We started them at 15 months.
The classes are usually pretty small so it can be tailored to the child's skill level and age. Right now, for example, there's my 25-month-old, a 22-month-old, and a 10-month-old in the class, so in one lesson we'll have the little one reaching out for toys in the water and the bigger ones jumping in off the side. I'd highly recommend it (and actually, there's a new session starting in a couple of weeks...).