For a three-act plot analysis, put on your screenwriter’s hat. Moviemakers know the formula well: at the end of Act One, the main character is drawn in completely to a conflict. During Act Two, she is farthest away from her goals. At the end of Act Three, the story is resolved.
Act I
The first twenty or so chapters of the novel take us all over the ship. We meet people of various levels of shadiness who ask for money, from the most sympathetic (Guinea) to the least (the herb-doctor and the stock broker). Things start to shift when we make the acquaintance of the cosmopolitan.
Act II
The cosmopolitan is the last confidence-man we meet—and the one we stay with the longest. Act II follows his exploits with a handful of misanthropes: Pitch (the Missourian), Charlie, Mark Winsome, and Egbert.
Act III
After Frank, the cosmopolitan, flips out at Egbert's inhumanity to man, he visits the barber for a shave he doesn't pay for then tucks an old man into bed. End scene.