Rogue Wave (Book Two) by Jennifer Donnelly

Six mermaids battle to save their hidden world. Serafina, Neela and four others have successfully escaped Captain Traho and his men after their hideout with the river witches was stormed. They must stop an ancient evil from escaping its icy prison by finding six talismans hidden by Serafina’s ancestor hundreds of years before. With all the Mer nations looking for them and no clue where to start, will the mermaids find the talismans before the monster is released?