For my commentary on this book, I'll reference the tags that Amazon asks you to choose between for the plot, mood, pace, and characters. Patrick O'Brian's work, and 'Master and Commander' in particular, is not served well by these kinds of choices. There are surprising plot elements, and ones that are foreshadowed long in advance. There are parts of the book that are light-hearted and hopeful, and parts that are very dark. The pace is slow as a walk on the island of Mahon, and fast as the seconds on the deck of an enemy ship.
In other words, O'Brian writes life. I can't give a better recommendation than that. Nailing him down as a sea-story guy or a history guy is a mistake. He writes friendships and hatred, hope and fear, and all the reasons and ways that we suffer and rejoice in little events and big. Read it.