Theories on the fish.

"But you haven't got the boy, he thought. You have only yourself and you had better work back to the last line now, in the dark or not in the dark, and cut it away and hook up the two reserve coils."

The Old Man is slowly being strung along by the fish, and he's even beginning to miss the boy. Now, with that said, it's technically Ernest Hemingway losing his will to live. The boy represents perhaps his own youth, innocence, and perhaps someone or rather, something else. I have no idea yet about the boy. There are several different things that I've come to associate him with, but nothing I can pin onto him yet.
But, I digress.
Well, we all know that he's in the ocean now without the boy, and he misses him dearly. He states it several times throughout the story when he is alone.
Perhaps that's a hint to his bi-polar disorder as well? Considering he's not always with the boy, and when he isn't he feels lonely and longs for the others presence, which is basically the core of his happiness.
He's always up and down with his emotions it seems.
Not only does the sea represent a mind, but the vast unknown laid out ahead of him, a journey.
Anyways...
I believe the fish is an idea too big for /him/ to even comprehend, or maybe it's his depression, but just alive and dragging him into his mind further. Perhaps it's both. Let's also keep in mind that the shore is reality. Now, with that said, let me say it in blatant terms without talking about The Old Man and the Sea. But, keep in mind that the Old Man and The Sea will apply to what I'm talking about here. Just keep whatever I talked about above in your head, and the connections will come to you with more ease.

Ernest Hemingway is getting more lost into his mind and no longer has the will to live. He only has himself, his depression, and his own huge ideas that HE doesn't even comprehend. He can't even put it into words. But, they're both only pulling him forward and leading him into the trap which is his own mind. It's consuming him, literally. The depression and the idea he's gotten has taken his person by whole, and he's slowly becoming more and more detached from reality BECAUSE they're both stringing him along to do so, and he's become obsessed with the ideas in his mind.
But that's just what I think so far.
I personally believe that Ernest Hemingway was losing his cool/mind near the end of his life.