This is a fantasy book from the mid-1980s to the early 1990s.  The story centers around a young girl who is raised as a servant by a witch.  The setting is the swampy area near a river.  Because the witch has told the girl she is ugly, when the girl comes in contact with other people (which is rarely), the girl wears a mask to hide her ugly face.  The witch calls the girl “Thingy.” In the story, the girl accompanies a stranger in a boat on the river.  She is escaping (?) the witch or acting as a guide(?) for the man.  During the journey, the girl learns her true identity and the truth about her appearance.