Why can babies breathe in water when there in the moms tummy but when they are born you cant breathe in water?

The unborn child does not breathe in their mother's womb. Instead they get oxygen dissolved in the blood supply from their mother through the umbilical cord (a sort of pipe which connects to the baby's tummy).

After the baby is born it can breathe for itself and the cord is not needed anymore and so is cut and tied by the midwife. This eventually shrivels up and falls off leaving just the "belly button".