Many times the water temperature can be matched to the body temperature, so that the baby is born into the same temperature environment as the womb, this reduces the shock to the infant (supposedly) and some say it makes for a more pleasant and natural birth experience.
Since the mother can move more spontaneously in water, there is greater comfort for her. In addition, with the opposition to gravity removed, the mother can conserve energy that can be used to cope with the contractions. It is reported that water relaxes the pelvic floor muscles, and water can reduce the need for drugs to stimulate labor, as women who have been placed in a warm tub have reached full dilation within an hour or two.
The usual argument is that there are two major benefits for the mother.
First, being immersed in warm water is supposed to reduce the amount of pain that the mother feels. This allows her to be more likely to deliver the baby without having to use drugs. The drugs dull the birth experience and are (some believe) bad for the baby.
Second, being in water is supposed to make it so that the mother is less likely to need an episiotomy and to make it less likely that the perineum will tear.
The feeling of control of the delivery process makes the mother more resilient and co-operant.
The period of time of water labor is shorter (approx. 6-8 hours) than in a normal labors (12-14 hours).
Since the mobility is raising in water environment, the delivery is much more comfortable. Also, the temperature of water (37 Celsius degrees) actuates the endorphin secretion. The endorphins are known as natural pain killers.
The water helps in avoiding the swelling of extremities of the mother during birth process.