An important moral lesson in The Old Man and the Sea can be summed up in the maxim "the journey is the reward."
Santiago uses courage, skill, determination, and the wisdom of long experience to catch the giant marlin, which is longer than his boat. Sharks eat all the meat, despite Santiago's best attempts to fight them off. Santiago enters the harbor with nothing to sell, only a huge skeleton. The important point, however, is that Santiago caught the fish. He fought the good fight with every ounce of strength he had and that is the true victory, not how much money he might have made. He has been true to himself and lived with integrity.
A related moral is Santiago's optimistic conviction that “man is not made for defeat. ... A man can be destroyed but not defeated.”
By "man," Santiago means what he believes is a true man: a person who faces his adversaries with all he has and doesn't give in. Santiago might have "lost" to the sharks, but he hasn't really lost because he has, all along, faced them with courage and honor.
Finally, another important moral of the story can be summed up in these lines:
Now is no time to think of what you do not have. Think of what you can do with what there is.
Santiago is an old man. He hasn't caught a fish in a long time, his boat and equipment aren't the best, and he isn't as strong as he once was. However, he lets none of this deter him as he faces his battle with the marlin and the sharks. When his harpoon breaks, he fights off the sharks with a piece of wood. He always makes the best of the resources he has at hand, knowing that success in life comes from relying on one's inner, not outer, strength.