In The Old Man and the Sea, Santiago is considered unlucky because he has failed to catch either the big fish. Part I of the novella is called "The Unlucky Boat" because he has gone 84 days without a fish to sell at market. Not only this, but he's old. The other fishermen equate old age with unluckiness: they think his mind and skills have left him. In short, they want him to retire. But, Santiago is not willing to compromise his fishing like the other fishermen (they fish for scraps, just to make money).
As you know, fisherman and baseball players are very streaky and, as such, very superstitious. They go through the same rituals (wear the same clothes) when on a hot streak. When they enter a slump, they will do just about anything to break out of it. This is why Santiago goes far out. He wants to salvage his career by going out on top, like DiMaggio. DiMaggio's father was a fisherman, so he's a perfect allusion to use. DiMaggio himself was known for his remarkable 56-game hitting streak.
So, to break out of his unlucky streak, Santiago risks it all for the big fish, way out, far away from land. He fishes during the riskiest time of year, September, when storms may swoop in and destroy his boat at any time. After all, he says, "Anyone can be a fisherman in May."