The Sea of Trolls by Nancy Farmer

Jack and his sister Lucy are taken captive by a ship full of berserkers, wild humans who raid and pillage villages. Jack, a magical Bard in training, sets off on a quest to save his sister and find their way back to their family. Dragons, trolls, and giant spiders are just a few of the obstacles that Jack must get past with a little help from a shield maiden, a crow and a giant pig.