The following contains spoilers for both the movie and the novels. If you wish to avoid the spoilers, my movie review in a nutshell is: Go See It. The movie succeeds brilliantly on its own terms and is respectful of O'Brian. It is, however, Weir's movie, not O'Brian's movie.
Starting with the movie as a movie, Weir has created a masterpiece. Though mostly scrubbed of gore, the scenes of 19th century war are convincing. Almost as good are the scenes of Surprise rounding the Horn. In this, and in showing the crowding of almost 200 souls aboard a small frigate, the movie succeeds in outdoing O'Brian. Though the movie is not at all a slavish adaptation of the O'Brian novel of the same name (the plot is taken from the tenth book and major parts of three other books find their way into the movie), O'Brian's major themes are sounded and a number of lines and sights are thrown in for no other reason than to please those who have read the series.
Weir's riskiest choice succeeds brilliantly. Rather than "opening up" the novel, Weir closes in on the Surprise and her crew. This is as non-commercial a choice as could be made. Rather than introducing a Hollywood romance, making the entire war depend upon catching the Acheron, or introducing the 19th century equivalent of a red timer ticking down to zero, Weir tosses out source material that might broaden the movie's appeal. O'Brian's The Far Side Of The World includes an adulterous love triangle, a mad gunner, his triple murder of the adulterous couple and (perhaps surprisingly on an 19th century man-of-war) an abortionist, and his subsequent suicide. In the book as in the actual 19th century, there were women on board English navy ships, both the wives of the warrant officers and even among the seamen. Rather than make one of these women the source of friction between Jack and Stephen, Weir (figuratively) tosses them overboard and focuses claustrophobically on the Surprise, the seamen and her Captain. This focus brings the audience to the final battle as a part of the crew.
Weir's real triumph is the choreography and filming of the battle scenes, which are done as well as any I've ever seen. Filming a general melee of three hundred men fighting for their lives with one-shot pistols, swords, pikes and knives in a confined space, Weir manages to present three or four themes in such a way that the viewer always can follow the action and tell what is happening to whom. At the same time, the audience feels the confusion and violence that the characters are feeling.
This triumph allows Weir to return to themes he has dealt with before, as early as Gallipoli, when he presented the insanity of World War I trench warfare as seen by Australian troops. This link comes through most clearly during the speech Jack Aubrey gives (most uncharacteristically) before the Surprise surprises the Acheron. Jack says that the Surprise is England and family and that the men will fight bravely for country and family, which of course they do. The Australians, on the other hand, were fighting and dying in an "European" war and, although they fought bravely, were fighting in the end only for each other.
Weir presents the deaths in Gallipoli as tragic and odd, where the deaths on the Surprise are presented as worthy and treated seriously but not as tragedy. This comes through in the choice of the identifiable characters who die on the Acheron; Nagle, Allen and Calamy. Nagle and Allen are not sympathetic characters. Calamy we are not allowed to know, though we are meant to like and admire him. His death (which is Weir's invention, not O'Brian's) is presented as coming during an opportunity he greatly desired and is the most bitterly regretted death in the movie. Soon after, the Surprise moves on and so do we.
In an interview about Gallipoli, Weir once said the following:
Our first approach was to tell the whole story from enlistment in 1914 through to the evacuation of Gallipoli at the end of 1915, but we were not getting at what this thing was, the burning center that had made Gallipoli a legend. I could never find the answers in any books and it certainly wasn't evolving in any of our drafts, so we put the legend to one side and simply made up a story about two young men, really got to know them, where they came from, what happened to them along the way, spent more time getting to the battle and less time on the battlefield.
The draft fell into place. By approaching the subject obliquely, I think we had come as close to touching the source of the myth as we could. I think there's a Chinese proverb - it's not the arriving at one's destination but the journey that matters. Gallipoli is about two young men on the road to adventure, how they crossed continents and great oceans, climbed the pyramids and walked through the ancient sands of Egypt, and the deserts of the outback, to their appointment with destiny at Gallipoli.
The end of the film is really all about that appointment and how they coped with it. I don't think we could have sat down in the early stages and got this - it took years of talking, writing, arguing, to finally get back to something incredibly simple.
The similarities with Master & Commander are clear. The differences are those between a younger man and an older man looking at life. Weir, at least in the context of the Napoleonic Wars, would betray his friend for his country. The theme from O'Brian's novel that comes through most strongly in the film is the conflict between the high Tory Aubrey and the liberal Maturin. Jack believes in the higher discipline; that men must be led both in order to accomplish anything worthwhile and for their own happiness. Stephen rejects this idea of man as a yoked beast. Stephen believes that power corrupts, and that's a shame for the powerful. The resolution of this dispute is perhaps the most disappointing part of the movie. Although Jack's idea of discipline wins out in the end, it does so only because he gives up the pursuit of the Acheron to save his friend's life. I think we are meant to see the need to blend the two philosophies in order to succeed (Jack and Stephen complete each other, blah, blah, blah), but we don't, because the Acheron reappears as a deus ex machina, with no connection to Jack's supposed sacrifice.
But perhaps this is the message, after all. The movie is almost entirely free of post-modern irony (the only exception, in which Jack wonders at this "modern age we're living in", is one of the movie's few clunkers). This earnestness leads to the movie's greatest surprise. Weir's movie is significantly more Christian -- at least, more explicitly Christian -- than O'Brian's novel. We are hit over the head with this at the end, with perhaps the only non-ironic, earnest Christian service I've ever seen in a major motion picture. Weir might think that Jack, as a Christian hero, is rewarded for his works, but actually he was rewarded out of grace.