MS. Found in a Bottle
We never get a name, but Poe sets up the narrator's basic characteristics right out of the gate. He stresses the character's lack of creativity—"a deficiency of imagination has been imputed to me as a crime" (1)—and a tendency to present things in boring, straightforward terms. He's not superstitious and he doesn't believe in wild stories, making him the perfect man to tell us this particular wild story straight up.
To Trust, or Not to Trust?
It also lends some credibility to the story, since he's not given to exaggeration. The narrator himself says as much to us: "I have thought proper to premise thus much lest the incredible tale I have to tell should be considered rather the raving of a crude imagination, than the positive experience of a mind to which the reveries of fancy have been a dead letter and a nullity." (1) Basically, what he's saying here is that we're about to read some crazy stuff, but he's not a crazy person, so we should take him at his word, and leave the grains of salt in the shaker.
But truth be told, that may be part of the satire. Think of it this way: after assuring us that he doesn't tell wild stories, the narrator then unloads the wildest story of them all—a tale full of fears, embellishments and emotional outbursts. The black galleon, for instance, appears on a wave a hundred times bigger than the ship itself. Can we believe what the narrator says? He assures us we can, but we need to decide that for ourselves whether or not he's reliable.
A Surly Sailor
Beyond his "just the facts ma'am" approach, the main thing to remember about this guy is that he's one isolated dude. He has no contact with his country or family, telling us that "Ill usage and length of years have driven me from the one, and estranged me from the other." (1), and goes to sea to fulfill some internal need (instead of traditional reasons like gold and sexy girls in hula skirts).
He speaks contemptuously of the crew of his first ship and points out rather snottily that the captain ignored his warnings about a hurricane: "he paid no attention to what I said, and left me without deigning to give a reply" (5). Yeah, because he's an expert meteorologist?
All this surliness comes back to bite him in the butt with a healthy dose of irony when the black galleon rears its creepy bow. Maybe his hopping on serves as a fitting punishment, since they can't see him and he is powerless to get the ship to change course. It's as if the world is saying to him, oh, you thought things were bad before? Well now you're even lonelier and in even direr straits. It might not be everyone's idea of just desserts, but you can't deny that it looks a lot like karma. The dude's found himself on a ship with the only people in the world more standoffish than he is.