Magic Below (Sequel to Bewitched in OZ) by Laura J. Burns
When Princess Ozma summons Zerie Greenapple and her friends Vashti and Tabitha, they have no idea what awaits. Tasked with infiltrating Glinda the Good's palace, Zerie travels by water, Vashti by land, and Tabitha by air. Zerie transforms into a mermaid and enters the underwater realm of Aquaria, where she is met with impossible obstacles. Not only is Zerie's concept of time put to the test, but soon her memory, values, and sense of self are also strained. Unsure whom she can trust in Oz, will Zerie's magic ultimately be used for good . . . or evil?