Its full title is Master And Commander - The Far Side of the World. Master And Commander is the first book in the series, and anyone who picks up the movie tie-in edition is likely to be confused, since most of the characters from the movie are either absent or much younger, and it's set on a different ship with entirely different events in the Mediterranean instead of the South Atlantic and the Pacific. The Far Side of the World describes events that have a least a superficial similarity to those in the book -- Jack Aubrey chases an American frigate (in the movie it's a French privateer, which is suspiciously large and heavily armed for a privateer -- can't have the Americans as the bad guys in a Hollywood movie) around Cape Horn and into the Pacific in order to protect British ships, especially whalers. However, the details are quite different in the book and the film.
So the film isn't really an adaptation of any of O'Brian's works so much as it is a separate story using some of the same characters and incidents -- enough of the same incidents that tamaranth and I were giggling helplessly long before Aubrey got to the punchline of the joke about the two weevils. Many well-loved characters have made it into the film: Pullings, Mowett, Killick (played with real relish by David Threlfall), Barrett Bonden (played by Billy Boyd, looking about two feet taller than I'm used to seeing him), Padeen, Mr Allen and many more are there. And they mostly seem to have their personalities and back-stories intact, for at least the amount that we're allowed to see (although Pullings' scar is only about 1% as hideous as it should be). Several of my favourite scenes are in there too. The trepanning of Joe Plaice. "May I trouble you for the salt?". The aforementioned two weevils.
And, of course, Jack Aubrey and Stephen Maturin are there. And they're both surprisingly well done. Russell Crowe is not big or blond enough to be Jack Aubrey, and his accent slips from time to time, but he brings some real vigour to the role. Stephen starts off slowly, and doesn't really come into his own until he is whisked away from the Galapagos islands without any time for a run ashore. We see Stephen the frustrated naturalist, and a little bit of the defender of freedom, but not the spy, the politician, the hopeless lover or (unsurprisingly) the drug addict. But there is hardly room in a single movie to accommodate all of his facets, and I think that Peter Weir and Paul Bettany did a fine job.
The film starts and ends at sea, with barely a foot set on dry land except at the Galapagos. We therefore see none of Jack's helplessness on land, or the deftness with which Stephen assists him. But again, it wouldn't all fit in a film, and I think it was right to concentrate on the nautical side of the books. The opening battle is very well done. The closing battle is rather more confused and less satisfactory, rather then being Jack Aubrey's great triumph. And the film itself ends on an ambiguous note, with plenty of room for sequels..
Rather like The Fellowship of the Ring, I suspect that it's close to being the best film adaptation that could be made of the original, while recognising that it's inevitably less than satisfactory in some respects. It certainly doesn't disgrace itself, and I think it does justice to O'Brian's characters and settings.