Santiago's skiff, his fishing boat, is also his entire world. He obtains his livelihood when fishing from his skiff. He stores tools of his trade (the fishing lines, the fish to be used as bait, the harpoon, the gaff, the net and sails) on his skiff when he leaves for a fishing trip. He carries food to care for himself - frequently caught from the sea and eaten raw - on the skiff. He sleeps on it, works on it, observes the world around him from it, and takes care of it so it can care for him.
When the skiff is threatened by sharks attacking his catch, he fights them off to the best of his ability, just as, in life, Santiago fights off those who would say he is too old and has run out of luck. The skiff allows Santiago to preserve his pride, showing all those in the village that he is still capable of catching an incredible animal and surviving a terrible struggle to bring it home. Despite tremendous difficulties, Santiago and the skiff will return to go fishing another day.