survivors says flipped ' after a commercial ship was spotted and men on deck .
hundreds of terrified women and children sheltering from the cold in the hull were trapped and died as water surged in and the boat turned upside down .
the commercial ship was unaware of the unfolding disaster and went on its journey , leaving men , women and children in the water before an italian naval vessel arrived at the site some 80 miles south of the island of lampedusa .