Water Witch — Connie Willis; Cynthia Felice

On the desert world of Mahan, he who controls water rules an empire. Mahali's rulers for generations were Water Witches, who could feel the ebb and flow of precious water in their very bones. But the royal family was slaughtered in a bloody coup, and control of Mahali's water passed to the impersonal hands of an immense computer network.

It was Deza's father who hit upon the scheme, dressing Deza in ceremonial garb and passing her off as the last surviving member of the royal house. Deza demonstrated water control with tricks and illusions, and in turn she and her father would be lavishly fed and bribed by offworld traders who dreamed of wealth beyond measure. When their tricks ceased to please, they would cut their losses and move on to the next con.

They were very, very good. But it's the nature of a con man not to know when he's about to get in over his head...