Another thing to think about (WRT advising women who are in labour) is that with the exception of premature labours almost ALL women want to meet the baby by the end of pregnancy.
A good midwife friend of mine tod me that you cannot keep a cooked baby in. So if a mama is keen to see baby but labour just doesn't seem to be moving forward much and mama could really do with rest but due to fear of slowing things down more is refusing to get into water even though she'd like to have the relief, it's worth asking why the labour isn't moving on much.
Some examples i can think of off the top of my head:
My friend - primip, 24, EDD+1, waters rupture at 9pm with slight (described as "not as bad as period pain") contractions 5-6 times an hour. Mama is too excited to go to bed, sure that intense labour is just around the corner. By 9am nothing has changed. She calls the hospital and they tell her to come in since it's been 12 hours since ROM. At book in she is 3cm dilated but barely effaced at all (and she has a very long cervix, about 5cm when fully closed, about 4cm at this point) and having painful ctx every 8 minutes. She is given until midday to progress, doesn't (everything is static), is given pitocin, progresses very slowly. By 3pm she has an epidural in place and after 32 hours awake she has a nap for an hour. At 6pm she is fully dilated and pushes with the epidural for an hour with very little progress. The epidural wears off and she is moved onto her knees and pushes for another 20 minutes. There is sudden progress and her son goes into immediate and persistent (i.e. not just descent and rotation distress-type tones) distress. The Ob rushes in with a ventouse, has mum put on her back, cuts an episiotomy and delivers him. He was asyclintic (head on one side) and not fully anterior (facing the back of her right hip instead of her spine), and had a nuchal cord and a nuchal hand, and had lots of bruises and marks from where he was against her bones and from the delivery. She is stitched up, loves him and they BF for 26 months.
Another woman i know: 2nd baby, 1st was medical induction for post-dates (EDD+10), 29, EDD+9, contractions begin around 10am but space out every time she relaxes. Eager to get the baby out and unconvinced (after the first birth) of her body's ability to get the baby out in a timely and safe fashion, she spends the next 12 house at home walking, cleaning, polishing, scrubbing, climbing up and down stairs and taking the toddler to the park. Her midwife (she is homebirthing) calls in around 8pm to see how she's doing and tells her to STOP. She prescribes a shower and a long sleep and charges the grandma (who is there to help out) with getting the toddler up and out or quiet the next day so mum can sleep in if possible. WOman reluctantly agrees but decides to give it a few more hours once midwife has gone. By midnight her partner and mum have ground her down and she reluctantly agrees to the bath. Bath halts labour altogether. Woman gets into bed at 1am and sleeps until 11am. At 11am she gets up, partner makes her toast, which she eats. She feels mild contractions while she's eating but nothing too exciting. At midday her midwife calls and they chat about how she's feeling and agree that she will call round again later in the day to listen to baby and see how mum is. At 1pm mum has a long, intense, very painful contraction. Immediately afterwards she vomits and asks for a bath to be run (to slow things down as it's so intense). Midwife calls back to mention something she'd forgotten and partner tells her what happened. Midwife arrives 25mins later to find mum pushing and baby girl is born 20minutes later in the bath.
There are definitely times when a labour will NOT get going until something which is not right (mum not rested, babe not in a good position) comes right. The best advice on this i was given was that if the mix was right and the baby was cooked no bath, shower, walk would keep it in. Once the process is coming on it is very hard to stop - think of all the premature babies who are born despite cocktails of powerful drugs. Really, how could a bath or shower prevent or slow something that is relentlessly imminent? Ina May Gaskin encourages women to go on with their normal tasks until they cannot and then do whatever feels good until the birth.
There might be times when as a midwife you will need to tell a woman that, keen or not, if her labour slows due to warm water or rest or anything else one would normally do, then maybe it needs to slow down for a bit!
Wow, mammoth post! :)