Although the old man, Santiago, loses most of his great marlin—all but the skeleton—to a school of sharks, he is not destroyed or defeated.
The old man badly needed a fish catch, as he had not had one in 84 days. Also, the great marlin would have earned him a great deal of money had he been able to sell the meat. However, people on shore are impressed with the size of the catch based on the bones of the carcass he drags back to land: they have never seen a marlin so large.
Further—and more importantly, as the story shows—Santiago fights the good fight to get and keep the great marlin. It's an exhausting and draining struggle to defeat the marlin. Santiago eats all his remaining bait to keep his energy up and hangs on as the rope cuts his hands. Once he has won the battle, however, the marlin is too big to fit in his fishing boat. Therefore, he has to drag it. A mako shark tears out a piece of it, and after that, the smell of blood brings many more sharks. Santiago does everything he can to kill and drive off the sharks, but there are too many for him.
Although Santiago lost the battle with the sharks, that is not what is most important. What matters is that he fought them with everything he had, physically, intellectually, and emotionally. This is what defines courage and a life well lived. It is not what you gain but how you live that is important. The old man can sleep peacefully when he is back on shore. He is undefeated because he has lived with integrity, given life his all, and is ready to go back out to sea the next day.