I agree with dymatsuoka, but I would like to add that it is something of a coming of age book about the transformation of Prince Caspian to King Caspian. In this book, he sets out on a trip which will define his reign as King. He " fulfills an oath he made to Aslan in Prince Caspian that he would, as soon as the situation in Narnia was secure, look for the seven lords sent across the Eastern Sea by the usurper Miraz." He is righting the wrongs of his late uncle and setting his house to order again.
He meets each test with courage and strength, and in most cases he is fearless. One of his biggest tests comes when he wants to go on to Aslan's country, and he struggles to remember that he must do the work which Aslan has called him to do. There is war brewing back in Narnia and he will have to be a strong leader for his people. By the end of the voyage, we see a young king who has proven himself in this campaign and is ready to lead Narnia. He realizes that he can not simply do what he wishes, but that he has been called to serve his people and he takes on the role willingly.
There are a number of important themes in the book, and I think one which is most prominent is the theme of redemption. This idea is best illustrated in Eustace, who, conniving and lazy, manages to alienate everyone on the Dawn Treader. Eustace is transformed into a greedy, selfish dragon, and through this experience undergoes a spiritual transformation when he learns to appreciate others. Eustace is ultimately redeemed by Aslan, who, through the baptism of a ritual bath, restores him to his original form, giving his a second chance at life.
Through the character of Lucy, two other important themes are explored - beauty and integrity. Lucy has always been sensitive about her plainness, and wishes she was comely like her sister Susan. She finds a magic spell in a book that will allow her to be beautiful, but remembers the wise face of Aslan just as she is about to cast it. She also learns a hard lesson about integrity when, because she has engaged in eavesdropping, close friendships might be forever altered.
A large theme throughout the book is Christianity. Although it is not directly stated, Aslan is undoubtedly a God/Christ figure. He resides in his own country, which is like heaven, and his presence permeates everything on earth. He is Redeemer to Eustace, and the Lamb who gives food to the spirit.