Poe is known for flowery descriptions, and a certain rhythm in his writing. It was one of the reasons why he made such a good poet. Since it's an early work, he's still honing his skills in "MS. Found in a Bottle," but the sentence structure focuses on descriptiveness, and there's a definite rhythm to the reading that helps the words roll off the tongue. Check it out:
The ship and all in it are imbued with the spirit of Eld. The crew glide to and fro like the ghosts of buried centuries, their eyes have an eager and uneasy meaning, and when their figures fall athwart my path in the wild glare of the battle-latterns, I feel as I have never felt before, although I have been all my life a dealer in antiquities, and have imbibed the shadows of fallen columns at Balbec, and Tadmor, and Persepolis, until my very soul has become a ruin. (22)
That first sentence is short and choppy—a simple statement with not much description. But that second sentence is a real doozy. It goes on and on and on, building clause of description on clause of description like a runaway freight train of imagery. He's a wordy guy, our narrator, and Poe wasn't afraid to stretch his prosey muscles.
Add to that his expected use of frightening imagery—including references to sea monsters, the howling of the winds and the bizarre qualities of the black ship—and you can see Poe's one-of-a-kind creepiness finding its footing.