Dark Tide (Waterfire Saga, Bk. 3).

Once a lost and confused princess, Serafina is now a confident leader of the Black Fin Resistance (BFR). While she works on sabotaging her enemy and enlisting allies for battle, her friends face challenges of their own. Ling is in the hold of Rafe Mfeme's giant trawler, on her way to a prison camp. Becca meets up with Astrid and learns why the Ondalinian mermaid is always so angry: she is hiding a shameful secret. Ava can't return home, because death riders await her arrival. And it is getting more and more difficult for Mahdi, Serafina's betrothed, to keep up the ruse that he is in love with Lucia Volerno. If Lucia's parents become suspicious, his life--and all of Sera's hopes--will be extinguished. Political intrigue, dangerous liaisons, and spine-tingling suspense swirl like a maelstrom in this penultimate book in the WaterFire saga.