One good example of symbolism is the sharks that attack the fish, strapped to the side of Santiago's skiff. Santiago, fighting them off, knows that they are not even hungry, but are simply responding to the smell of blood in the water:
They were hateful sharks, bad smelling, scavengers as well as, killers and when they were hungry they would bite at an oar or the rudder of a boat... they would hit a man in the water, if they were hungry, even if the man had no smell of fish blood nor of fish slime on him.
(Hemingway, The Old Man and the Sea, Google Books)
Santiago considers the sharks to be sub-creatures, with no purpose beyond killing and destroying. He has done an amazing thing, caught an enormous fish, and these sharks care nothing for his achievement except that it easily feeds their own hunger. In the sharks, Hemingway symbolizes people who have no ambition or drive to succeed, but wish only to tear down people who have achieved something. Those who fall under this category live in endless envy of ambition and creativity, never willing to create or achieve for themselves but only desiring to destroy any works that are popular, visionary, or creative. The sharks, in their mindless drive for destruction, showcase this anti-ambition.