In The Old Man and the Sea, why did Santiago want the marlin to turn and swim with the current?

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:

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"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.