Hours after Santiago hooks the marlin, Hemingway describes the fish pulling Santiago's skiff with alliteration:
But four hours later the fish was still swimming steadily out to sea, towing the skiff, and the old man was still braced solidly with the line across his back.
There is a repetition of /s/ sounds throughout the sentence, specifically in “still swimming steadily,” “sea,” “skiff,” and "still braced solidly." The alliteration is an instance of sibilance, and it sonically mimics the hissing of water as a boat cleaves through it. More importantly, the /s/ sound emphasizes the strength of both the fish and the boat by drawing attention to the phrases “still swimmingly steadily” and “still braced solidly.” This instance of alliteration appears relatively early in the story: the marlin will pull the boat along for days—and Santiago will hold on all the while—although the reader does not know as much yet. Regardless, both Santiago and the marlin have untapped reserves of stamina, and Hemingway highlights both of their strength early on in their tête-à-tête by drawing attention to "swimming steadily" and "braced solidly."
The alliteration also equates Santiago and the marlin. Long before Santiago calls the fish his brother, the narrative uses the same sound to describe both Santiago and the fish he is battling, thus drawing an equivalence between them. This comparison is later made explicit.