I've had two homebirths but on dry land so can't help with the pool sorry.
We used cheap shower curtains and bought an Asda basic fitted sheet to secure them to the bed (nice to lie on too), other things I had were maternity pads from Boots (MWs never have enough inco pads), loads of old towels (my lovely friend took them home and washed them all they were meant to be for throwing out) and some very soft kitchen towels. That was about it.
Hospital supplied the rest, gas and air canisters, delivery pack etc
Oh I had to get a presciption from my GP for diamorphine (didn't take it), Stemitil (for sickness) and co-codamol for afterpains.