The general argument in favour for water births revolve around giving the newborn baby a less traumatic entrance in the world. I have read (in magazines, not scientific journals) that the transition from amniotic fluid to water interface is more comfortable for the baby than the transition from amniotic fluid to air.
Whether that is true is unknown- it’s pretty hard to know what level of comfort a newborn baby is in.
What is known is that there are no clear benefits to having a water birth, but there are a number of documented, potentially fatal consequences of having a water birth.
During childbirth, the bowels and bladder will move due to the use of pelvic muscles trying to push out the baby, as well as the movement of the baby pushing on the bowels and bladder.
This means that if you’re in a bath or pool of water that isn’t being drained, the water will fill with your faeces and urine.
In the uterus, a baby’s lungs are filled with amniotic fluid. When they are born they need to expel this fluid and fill their lungs with air. Babies have two reflexes to help them make their lungs start working- the cold crying reflex and the cold pressor reflex (more information found here and here). Basically when a baby enters an air interface, their skin cools and this triggers a coughing/crying response to get rid of the fluid, and also increases the blood pressure to the lungs so blood can be sent there to be oxygenated.
In order to cough, you need to take a breath in first.
If a baby is submerged in water filled with bacteria from their mother’s faeces, they could ingest bacteria into their lungs.
Now looking at the literature, there are few good scientific trials out there to conclusively evaluate water births. The studies that have been completed however show that there is an increased rate of near-drowning, pneumonia, hyponatremia inducng seizures, and respiratory distress in babies born in water births as opposed to a normal birth. There have even been deaths directly attributed to water births, such as when those aforementioned conditions become fatal (source).
Are there any benefits to having a water birth? A study showed that mothers feel more satisfied with their pushing efforts.
If you or any of your friends are thinking of having a water birth, I would advise them against it unless they are part of a controlled trial. Whether at home or in the hospital, having a baby on a dry surface, so the baby can be born with air ready to fill its lungs, will give the baby the best chance at a happy, healthy start to life!