Laura J. Salgado 
Mission [EPUB ebook] 
Green Island

Christy is thrilled to join the Pirate Girls, an elite team serving the King who lives in the magnificent castle up the hill from her town. Ignoring ominous warnings of danger, she boards the Golden Cross on a mission to rescue Melissa, a girl being spritually attacked on Green Island. During the voyage, dark forces oppose the ship making Christy doubt her abilities. When she finally recognizes the attack of the enemy, she worries she has done too little, too late.