Since it occurs in the title, the Sea is as much a full character as the Old Man Santiago or the Boy Manolin. This is the place where Santiago has chosen to live out his life, and the place which he knows as well as any person; the Sea has moods and temperaments, and it is in the interpretation of the Sea that Santiago can come to terms with both his own mortality and his lasting legacy.
The clouds were building up now for the trade wind and he looked ahead and saw a flight of wild ducks etching themselves against the sky over the water, then blurring, then etching again and he knew no man was ever alone on the sea. He thought of how some men feared being out of sight of land...
(Hemingway, The Old Man and the Sea, Google Books)
Santiago has come to terms with his life so fully that even if the Sea were to take him, he would not be sorry, nor would he blame it. Each event of the story rests on the Sea and the life inside it, and each action of Santiago rests on his love of the Sea. Had the story been merely a metaphor, without Santiago's fierce devotion, it would be nothing but a fishing story; instead, Santiago's entire life is slowly revealed through his interactions with the rippling facets of the Sea.