If a woman's water breaks suddenly, does an ambulance have to be called?

Unless there's blood in the water, not necessarily (more or less)... If you're quite far from the hospital, better arrange your own transport so as not to waste time (except maybe if the ambulance is parked somewhere near your place). If the baby's head is not yet out and you have a written letter from your wife's obstetrician, go directly to the delivery room/dept and hand the letter in one of the nurse's there so that they can contact the OB. However, if the head is already coming out, head straight to the ER. At least this is what we do in our country that is still developing...