You can't have a sea story unless you go to sea, and "MS. Found in a Bottle" begins with an explanation of why the narrator finds himself on a boat: namely, that he doesn't like his family or his country, that he has money for traveling, and that he feels an inexplicable restlessness that takes him to sea.
Things properly get underway with the hurricane that swamps his ship and eventually sends it straight in the path of the black galleon. Poe constantly emphasizes the danger here: the narrator could be swept overboard and drowned at any time, and we're left in suspense as to whether or not he'll make it.
The danger diminishes slightly on board the black galleon, because the creepy crew seems to know their way around a ship. But the sense of mystery remains. Where are these old men going? Why are they in an older ship with ancient instruments? And why are they so stinkin' surly?
The narrator learns of the galleon's destination: a giant whirlpool at the South Pole. Um, what? Yep, that revelation is the high point of suspense, excitement, and plain old-fashioned terror in the story.
The narrator explains his horror as the ship slowly circles the whirlpool. Falling action? You betcha.
The ship gets sucked into the whirlpool with the narrator onboard. Their final fate is unknown, but we're betting they're not relaxing on a beach in Bora Bora if you know what we mean.