We are starting this week! We went to watch a baby class last week though. My DD is actually 2 years 4 months, and she has swum in a pool and in the sea on multiple occasions, but we have never been to classes before, and she will be going to a baby class with babies as young as 6 months.
Anyway, a lot of the lesson that we watched was, as PP said, just getting them used to the water and building their confidence and so on. They played games and sang songs. So it isn't really teaching them to swim, and since my DD is already reasonably confident in water it might all be a bit pointless, but we have a free day with nothing to do and thought it looked like fun!
All of the lessons near us take kids from 6 months (not younger) and I believe that they are supposed to be fully immunised to that age before starting - not sure if they check but I know that several people have told me that.