At the beginning of the novel, when introducing readers to Jim, Conrad describes Jim’s fantastical visions of himself as a hero, using alliteration in the process:
He saw himself saving people from sinking ships, cutting away masts in a hurricane, swimming through a surf with a line; or as a lonely castaway, barefooted and half naked, walking on uncovered reefs in search of shellfish to stave off starvation.
Conrad’s alliteration—which in this case counts as sibilance—is centered around the /s/ sound and appears in the beginning, middle, and end of the passage: “He saw himself saving people from sinking ships […] swimming through a surf […] in search of shellfish to stave off starvation.”
Conrad’s decision to use alliteration in this passage is intentional—this stylistic choice adds a poetic quality to the language that matches the romantic content of the words. In other words, the alliterative element helps readers understand that these visions Jim has of himself “saving people from sinking ships” and bravely surviving as a “lonely castaway” are just that—visions—and not necessarily grounded in reality.
It is notable that, later in the novel, when Jim is confronted with the reality that he is not someone who saves people from sinking ships, Conrad does not use this sort of lilting, lighthearted alliteration.