As the others say it is something you discuss at your birth plan appointment at 36 weeks although you can obviously always change your mind on the day to not go in, or to give it a go, they don't reserve the pools so it's usually just a case of luck as to whether it's available. I had casually mentioned it as a form of pain relief but wouldn't have pushed it on the day, but thankfully a lovely MW (who hadn't even seen my plan yet) asked if I wanted to get in, best thing I did. Doing it at home this time hopefully to get a close as I can to a "guaranteed" (in terms of availability at least!)