Again and again in The Voyage of the Dawn Treader, characters think they're up against a terrifying magical enemy, only to discover that this enemy is an illusion; the true enemy is themselves. It's a case of "we have seen the enemy, and it is us." In this respect, Voyage of the Dawn Treader is different from the other six Narnia chronicles, each of which has a clear and present antagonist: the White Witch in The Lion, the Witch, and the Wardrobe, King Miraz in Prince Caspian, the Lady of the Green Kirtle in Silver Chair, Empress Jadis in The Magician's Nephew, and so on.
Part of the reason Voyage of the Dawn Treader is so different is that it is an episodic novel with lots of mini adventures in it. In each adventure (usually one per island), Caspian, Eustace, Lucy, Edmund, and their friends must face a new and dangerous situation. Yet, with a few exceptions, most of these situations are really about facing their own fears and emotions.
For example, Eustace's transformation into a dragon is simply a magical manifestation of his personality – his selfishness and his layers of self-protection. In order to defeat the enchantment, he has to decide to change and cultivate a different attitude. Similarly, when the adventurers discover water that can turn anything into gold, Edmund and Caspian must overcome their own greed and pride. During the adventure on the island of the Dufflepuds, Lucy thinks she is facing an evil magician, but she is really facing her own fear of the unknown. And near the end of the book, the entire crew of the ship must cope with the dark island where their worst nightmares become real. Oh, sure, there are some traditional antagonists too – a monstrous sea serpent here, a corrupt politician there – but for the most part, The Voyage of the Dawn Treader is about fighting and gaining mastery over yourself.