SPOILERS for The Far Side of the World; here’s the non-spoiler post if you got here by mistake.
Okay, two major differences from the movie:
Women. The movie has one (1) visible woman (with a parasol in a boat) and one (1) referenced woman (Sophie, in a letter). In the book, women are very important, possibly as important as any female character who isn’t Sophie or Diana. Mrs. Horner, the gunner’s wife, makes the whole Hollom-Jonah subplot much more complicated and tragic; and of course there are the would-be Amazons.
(I have no idea whether they are based on anything historical. The appearance of native savages makes me automatically twitch, but I didn’t have any specific problems with the details of the portrayal.)
The fate of the enemy ship being pursued. It’s much more anti-climatic, but it’s certainly realistic, and Jack can’t defeat every ship he pursues. And there’s more than enough excitement in the time on the island with the Norfolk‘s crew, which was perhaps nodded at by the ending of the movie.