Who is the narrator, can she or he read minds, and, more importantly, can we trust her or him?
Third Person (Omniscient)
Melville's narrator is a tricky devil. Having access to the consciousness of multiple characters gives this narrator a lot of power and allows him (her? it?) to inform our understanding of everyone and everything in the novel. We get a hefty survey of characters, and we end up with a good idea about their suspicions, prejudices, hopes, and fears.
The curveball is that the narrator doesn't tell us everything he knows. Instead, we get moments when the narrator wonders aloud about what's going on in various characters' heads—even though we know he must know. For instance, take a look at this passage where the narrator tries to guess what Guinea is thinking during that awful coin game:
To be the subject of alms-giving is trying, and to feel in duty bound to appear cheerfully grateful under the trial, must be still more so; but whatever his secret emotions, he swallowed them, while still retaining each copper this side the oesophagus. And nearly always he grinned, and only once or twice did he wince, which was when certain coins, tossed by more playful almoners, came inconveniently nigh to his teeth, an accident whose unwelcomeness was not unedged by the circumstance that the pennies thus thrown proved buttons. (3, 18)
An added bonus is that our narrator addresses us directly, even if he never really enters full-out second-person narration. Woot. He's sly about it, of course. He doesn't say things like, "You're probably wondering why I did x just then." Instead, we get chapters that offer commentary on the book. These moments kind of make us conflate Melville's narrator with Melville himself, since they read as sort of manifestos of writing fiction.
Basically, Melville's narrator is the way he is because Melville wants you to know a lot about what's going on—but so not everything. There's supposed to be a little mystery. The book is supposed to be weird. You're not always supposed to know what's going on.
So just roll with it and laugh along with—or be creeped out by—Melville and his wacky cast of weirdos.