A happy nappy is just a particular brand:
https://www.friendlybaby.co.uk/swimming/splashabout-happynappy.html?gclid=CNXRkZicyZwCFV8B4wod7SIAKw
From the swimbabes website (who we are having lessons with):
"Babies and toddlers attending our course must always wear two aqua nappies whilst swimming: a disposable swim nappy (such as Huggies), together with a Happy Nappy on top. The Happy Nappy has been specifically designed for baby swimming programmes such as ours whereas other swimming nappies can leak due to failing to fit snugly enough around babys legs."
I would rather use a reusable, but I really want to do the lessons, so it's ok.