I was in labour for about 16 hours (that's what my notes say, but the first 8 hours were relatively pain-free and I spent it at home with a TENS machine attched to my back (I quite liked the TENS by the way which I had hired to use at home).
Once I got to the Edgware Birth Centre (North London - NHS - free of charge - 5 rooms 3 of them with a private pool - I loved the place !) I was around 4 cms dilated so they let me get into the pool straight away. I was hoping for a water birth, but things just went too slowly. Plus my first midwife was a bit too laid back. The 2nd midwife (thank goodness for her arrival as I think I would have ended up in hospital without her) made me get out of the water.
Baby was delivered within 50 minutes of the 2nd midewife arriving - on land on a Dutch birthing stool (it looks just like a toilet !). The water DOES help. You notice it the second you get out. I did use gas/air (the only pain relief available when you are in water), but it is purely psychological, and it didn't really help much.
You don't have to get out of the water to be checked. They only did one vaginal check and that was when I arrived. The other checks are with a long stick that has a mirror attached to it and a torch - as I was standing in the water the midwife could look for signs of crowing with the mirror.
As my DH was with me the whole time, he helped with adding water. I asked for warm water whenever I felt it was getting too cold. But as there is loads of water, this doesn't have to be done too often. The plastic lining of the pool kept the temp. warm most of the time.
I was extremely lucky (and studies indicate that water can prevent tearing)- I had no tearing and no stiches. The water keeps your skin elastic and if you start doing pelvic floor exercises and massaging the perineum a few weeks before you are due (I used olive oil to massage my perineum before I was due - or get your DH to help you if you can't reach down because of you bump !)
I don't think I could have done it without the water, as DS was my first and I was 37 - so no spring chicken . I highly recommend it.
Good luck.