What is the theme in The Thirteen-gun Salute by Patrick O'Brian?

Life at Sea is a theme. The novel presents two communities of seagoing individuals—one aboard Surprise and one aboard Diane. Similar in many respects, the two groups have distinctive traits—one is comprised of private sailors aboard a privately owned ship; the other is comprised of Royal Navy seamen aboard a man-of-war. The novel does an excellent job of contrasting the two communities. For example, aboard Surprise there is much singing, chatting, and frequent laughter while on Diane these things are not countenanced. Surprise is crewed of men seeking personal fortune whereas Diane is crewed of men seeking to carry out a mission with little chance of remuneration.