I had my heart set on a waterbirth after seeing a video of it in sex ed aged 10 (!) - I think we must have had a very unusual sex education video 
Anyway - I went in the pool with DS1, absolutely loved it, brilliant, it dialled the pain down to about 10% of what it was and brought it from unbearable to manageable again. Added gas and air when it got towards bad again. Just lovely. So calm and would definitely repeat. They got me out towards the end to check me and I ended up giving birth on dry land but still definitely worth it.
DS2 was 10 years later, in another country and I had forgotten everything, I think he might also have been back to back, he was an odd labour. I wanted to get into the pool again so I asked for it and was given access to it, but it didn't really help and I was disappointed and felt discouraged and asked for an epidural instead. What I learned later is that I was only 1cm dilated when I got in and I shouldn't have really been given access to the pool that early - this wouldn't happen in the UK hence the misunderstanding. That was not the best birth as epidural didn't work properly and it all got a bit hectic and scary.
DS3 3 years later than this. I was really worried about repeating the experience I had with DS2 so I agonised and analysed both birth stories for ages and then just realised that what I needed was WAY more coping strategies for early labour to hold off for as long as possible getting into the pool, and to ask my midwives to feed back a bit more as the midwives I had with DS2 were really vague and uncommunicative and I kept assuming things that turned out to not be what they said/meant at all.
So anyway, when it came to labour, I was doing all my gravity-harnessing, best coping strategies, and waited for as long as I could stand before getting into the pool, and again, it was magic just like it was with DS1. However I didn't stay in it for as long. It was good and it enabled me to really go into a calm zen space and breathe through several contractions and probably progress for a couple of hours, and then I asked for gas and air as well to be told they don't let you have it in the pool... FFS! (Again, not UK. In the UK they let you have it in the pool.) So I got out in order to get my gas and air. The end was bearable with the gas but I was fed up and tired and just wanted to get through it. All fine. Happy enough with the birth - would have changed a couple of the red tape things.
In hindsight, my tips would be:
- If you have a choice of hospitals, ask them both/all what their pool availability situation is. The reality is waterbirth isn't popular in the UK and the pool usually will be available, but of course more availability is better.
- Do a good course or learn as much as you can about the physiology of birth - I found it SO helpful to be able to visualise and understand what was likely to be happening at each stage and what I needed to do to maximise the effectiveness of each contraction. If you're in the midlands, my old teacher runs Birthability and she was fantastic.
- Do a good course and/or collect as many early labour coping tips as you can find. Juju Sundin's Birth Skills is excellent - I really really rate this. There are also several great pregnancy youtube channels with birth tips.
- Delay going into the pool until you are at a state where you feel like you're almost at the edge of not being able to stand it. Don't get in too early, it works best if you're struggling to cope as it brings you back a few notches.