While the marlin pulls on the skiff, it is swimming either with, across, or against the current of the sea. If it is swimming against the current, that shows that it is still strong and able to pull, meaning that it would more easily escape if Santiago tried to pull it in and kill it. However, if it turned to swim with the current, that would show that it is tiring, trying to swim more easily instead of pulling the heavy skiff. The narration states:
"He's headed north," the old man said. The current will have set us far to the eastward, he thought. I wish he would turn with the current. That would show that he was tiring.
(Hemingway, The Old Man and the Sea, Google Books)
Santiago also wants the marlin to tire because his hands are being injured by the line; he cannot tie it off, because a sudden jerk will snap it. Instead, he has to hold the line taut and feed it when there is tension, and so the pull of the marlin against the current hurts. Unfortunately for Santiago, it takes a long time for the marlin to tire out.
as we have seen in the novel , the old man was very tired and pain in his hands that are cut when he was pulling the larg marlin , so he wish that the marlin turn and swim with the current at least he might rest for a while to regain his power and eat