Santiago views the sea as a creature all of its own; it reacts to human activity in motion with its own moods. The narration treats the sea as entirely neutral; it cannot think and so it only moves with the tides and weather. Santiago, however, has lived on the sea so long that he has humanized it:
Some of the younger fishermen... spoke of her as a contestant or a place or even an enemy. But the old man always thought of her as feminine and as something that gave or withheld great favours, and if she did wild or wicked things it was because she could not help them.
(Hemingway, The Old Man and the Sea, Google Books)
In other words, the sea is entirely a thing of instinct; it can help or hinder at random, and while its help should be appreciated, its hindrance should not be decried because, again, it does not care who is affected. Santiago respects the sea as a force of nature and as a personal acquaintance, and he knows that if he were to die at sea, it would be acceptable, because he has gained his entire life from the sea in the first place.