In the book, Santiago is an old fisherman who has had a bad case of luck; he has gone a long time without catching any fish.
When he does manage to catch the marlin, he is far out at sea. By this time he is tired and weak and unable to make his way
back home. When the sharks come and eat the marlin, he realizes that it was all for nothing. He reached his goal of capturing
the marlin, even if it was eaten by sharks, so he was defeated but not destroyed. When Santiago says "I am sorry that
I went too far out. I ruined us both," to the marlin, the literal meaning is easy, but I believe it figuratively says
that Santiago went on fishing too long and it was the end of him. He was old and so capturing the marlin took a lot of his
strength, so when he "went too far out" it figuratively means that he went on fishing alone too long.