I took both of mine after the first set of jabs. It may have changed but don't go within so many days of polio vaccine, It is without a doubt the best thing I could have done they are both very strong competitive swimmers. Its all about gaining confidence at this stage which you can not get in a bath or paddling pool, very different environments.
When you first go, go with a friend/partner with you so you can maybe have a bit of a swim yourself first while they hold LO, then literally bring baby in for the last few minutes. You won't do any more than 5/10 minutes for quite some time as they lose heat quickly.
Move her around in the water, get a little rhythm going 'on your front' on your back' and just swoosh her gently around. Then while holding her blow a few bubbles in the water. As she gains confidence don't be afraid to swoosh her under water for no more than a second or two. Babies do have a natural instinct to hold their breath under water. DD was able to paddle between DH and I unaided very quickly doing this.
Then get out and have a towel poolside so you can wrap her and get her warm.
Make sure you have more than 1 spare nappy afterwards (I speak from bitter experience) especially if a DD as they wee so much afterwards and your nice clean, dry nappy will be full before you have got dressed yourself.
Have loads of fun, Within a few weeks she will be ready to go along to a mum and baby session and play with the toys. Try and go to a slightly quieter session to start with.