What is the theme in The Surgeon's Mate by Patrick O'Brian?

Friendship is a theme in the book. The two protagonists of the novel, Jack Aubrey and Stephen Maturin, share the spotlight nearly equally. The two men have enjoyed a prolonged friendship which has developed slowly over the course of six previous novels. Their mutual confidence has survived conflict, suspicion, and even competition for the attention of a woman, and emerged the stronger for it. Both men are able to haltingly share their innermost fears with the other, and their natures are such that their respective spheres of expertise complement the others' without conflict. Of course, this is limited with respect to Maturin's intelligence work, though Aubrey admits the work is important, finds it distasteful, and ignores it insofar as he becomes aware of it.