How is the setting of The Old Man and the Sea important to the plot?
The Sea acts as the main setting of the novel, and is shown as an uninvolved force of nature, not concerned with the actions of the living things in and above it. The deep depths are a matter of concern for Santiago, as he fears the marlin will dive and either snap the line or sink the skiff, and as he travels farther from shore, Santiago worries about returning home safely. The Sea helps him, soothing his burning hands, but also hinders him, hiding sharks that attack his catch:
They came in a pack and he could only see the lines in the water that their fins made and their phosphorescence as they threw themselves on the fish. He clubbed at heads and heard the jaws chop and the shaking of the skiff as they took hold below.
(Hemingway, The Old Man and the Sea, Google Books)
Every action Santiago takes is informed by his experience on the sea, and he doesn't resent it for its uncaring nature, just accepts that it is more powerful than he is and that he should be wary and grateful that he can survive on its surface. Without the vast sea, the story would just be "The Old Man," and so have no impetus. His mastery of sea fishing is his drive; the sea itself is his destination; the fish in the sea are his goal; in the end, Santiago defeats the sea in staying alive, but leaves part of himself behind, deep in its rolling waves.