Just put her in – she won't mind.
We have a Jojo Maman Bebe inflatable ring with neck support which works well.
We also take our girls out and hold them/swish them around etc.
I have stopped short of dunking under but I want to as they are meant to be fine with it.
We have taken ours since around 3 months and they have loved it every time.
Make sure you go to a proper infants group though, not a parent and child one with 5 yr olds jumping around.
EDIT: Agree with above – 15/20 minutes will be plenty enough time and make sure you take her towel to the poolside and wrap her in it straight away.
And don't forget the swim nappy!
Finally – Jojo also do the neoprene body suits which help them keep warm.
🙂