There's nothing natural about it. Even in the animal world, every air-breathing creature gives birth or lays eggs on land whenever possible. Even sea turtles and seals, who live happily in the sea, take great pains to deliver their young safely on land. Human babies born underwater face unnatural health risks no baby should have to face. And their mothers face unnecessary risks, too.
Babies sometimes breathe the moment their head delivers.
I once caught a baby who poked his head out of his mama, promptly breathed in, then sneezed loudly. Fortunately he had good clean air available. This is life-threatening and usually can't be prevented. In a natural birth, the uterus and birth canal are flushed out by amniotic fluid, the passage of the baby and finally the placenta. Any foreign material is naturally washed away from the body.