What is the logic of the old man going so far out in the sea?

In The Old Man and the Sea, the old fisherman Santiago becomes obsessed with breaking his unlucky streak. Then he encounters a massive marlin that pulls him very far out from the shore. Out on the sea, far from the safety of his little shack, Santiago experiences the beauty and the horror of the sea and of the world as a whole.
Santiago's excursion far from the sea is more allegorical than realistic. While it's possible that a giant marlin could pull the man out on his boat, it seems fantastical and almost hard to believe. Hemingway's typical style usually follows a firm sense of realism, and so this novella is certainly an exception.
On the other hand, from a character standpoint, there is nothing about the situation that doesn't make sense. Santiago is obsessed with breaking his unlucky streak; he also has a deep appreciation and admiration of the sea, which is shown in his belief that nobody deserves to eat the esteemed marlin. Santiago ventures out to sea because he has nothing else but his love of fishing and his appreciation for the wilderness.