Attended Waterbabies classes with DS2, as something to help with the binding recess after I'd had quite bad PND.
I loved it, DS2 loved it. Was a lovely experience, and I have some great photos from the organised underwater photo session.
The underwater bit is only a small part of the class.
I still get a warm glow when I think about supporting his head with be hand whist the other hand was the twinkling stars as we sung twinkle twinkle. He was so calm and fascinated by it.
It is however, very expensive.
And I echo the point above, nice warm pools, but changing rooms are often not designed for 10 mums with babies to get changed - so expect, cold, cramped changing rooms and having to change your baby on a wet, sometimes not so clean floor. I just sucked it up - took a waterproof, padded chasing mat and a couple of spare towels, and kept DS2 in his car seat whilst I got changed.