'It's an odd thing about you, Stephen,' said Jack Aubrey, looking at him with affection. 'You have been at sea quite some time now, and no one could call you a fool, but you have no more notion of a sailor's life than a babe unborn.'
And here we have an authorial recognition of part of Stephen Maturin's function in these books (at least the first two): he doesn't know the things the reader doesn't know about sailing ships. Some of it gets explained in authorial voice, but there are bits that other characters can explain to the good doctor because he's the designated outsider. He's the equivalent of the time traveler, the alien, the stranger in a strange land -- which can be thematically interesting, to be sure, but it's also extremely functional.
And here's the thing that interests me: I don't mind seeing the strings. Sometimes I get extremely frustrated when the author telegraphs too clearly what a different element is being used for. "Yes, yes, it's a recurring image," I'll mutter, "how happy for you." Or, "Hurrah for Captain Exposition."* But when Dr. Maturin makes Captain Aubrey serve as Captain Exposition, I don't mind it.
I think it's because Dr. Maturin has other traits, and because this one makes sense in the context of the rest of his character. O'Brian has established him as the sort of man who notices a great deal about some things and nothing whatsoever about others, and so it all fits. Even if you know how it all fits, you don't feel that Dr. Maturin's toes have been cut off to fit into Cinderella's slippers -- so it doesn't matter if his shoes show up from time to time, because they're his.
And before that dreadful metaphor snaps entirely, I'd better go do something else and pretend I didn't say it.
*This, my dears, is the difference between an amiable reader and an uncritical one. I'll read just about anything, given half a reason. But I will not refrain from muttering about it.