Glinda of Oz [The Wizard of Oz series #14]

Peace, prosperity, and happiness are the rule in the marvelous Land of Oz, but in a faraway corner of this magical domain dwell two tribes--the Flatheads and the Skeezers--who have declared war on each other. Determined to keep her subjects from fighting, the Ruler of Oz, Princess Ozma, along with her dearest friend, Princess Dorothy Gale (formerly of Kansas), embarks on a quest to restore peace. When the Supreme Dictator of the Flatheads refuses to cooperate with Ozma, she and Dorothy seek out Queen Coo-ee-oh of the Skeezers, hoping she will be more reasonable.
But the queen imprisons Ozma and Dorothy in her grand city and then traps them by submerging the whole city under water.