Many midwives and doctors who have worked with water births see a significant correlation between entering the bath after the woman's cervix is dilated beyond five centimeters and a reduction in the remaining length of labor. It seems that if a woman waits until her labor is progressing well before getting into the water, then the contractions are more effective and complete dilatation is often accomplished faster.
Water birth, although a desirable choice for many women, is simply an option, not an end to be achieved in every birth. When women have the birthing tub available, many gravitate to it without hesitation and feel safe staying in and giving birth to their babies. If everything is proceeding normally, the decision about when (or whether) to get in and out should be left up to the mother.
For more information, see our article on water birth.