Unpredictable Parts in The Old Man and the Sea

In the book the  Old Man and the Sea the author was stubborn. Reading was boring and uneventful. That was until the old man was on the sea finally was able to reel in his monster of a fish. This "marlin" put up an amazing fight for just a fish, tugging the old man's boat for days, then surfacing momentarily only to go back under to pull the boat even further. I do not think it was not actually a fish. I think it must have been either an illusion put on by Santiago's own mind or an anchor that had come apart from its ship and was hooked accidentally. In the case of the anchor, the old man must also have been insane to think he saw it swimming. The current system in the ocean must have moved the anchor, which then also tugged the boat in a whole. Supposedly, Santiago was also able to hold onto the swimming fish with only his hands clamped to the line for dear life; it seems completely impossible for anything to hold onto anything voluntarily for too much more than about half an hour. Even if Santiago did have this strength, I question who he controlled the fish. The question remains without an answer. Or why would he want to stay with the fish for so long, knowing he was slowly being pulled farther and farther away from home, making it harder on himself to get the beast back into shore? It sounds quite unrealistic. The author, Ernest Hemingway, should have asked himself these sorts of questions before finishing the book, but that is just my opinion. Still, without understanding the flaws in the story, the plot in entirety was alluring but somewhat predictable. A man went out to find the catch of a lifetime, found it, fought it, and was unavoidably doomed from the start of took more than he could handle. In the end the marlin was unrealistic.