Quote 19
Despite overwhelming odds against him, the old man never gives up until the very end.
The old man recognizes that, although practically there is nothing he can do, he will still fight for the sake of enduring.
Quote 20
Although he "knows the fight is useless," the old man continues to battle the sharks anyway. This suggests he is fighting for some other motivation than "winning."
When Santiago’s strength fails him, he simply waits patiently for it to return.
Quote 21
All my life the early sun has hurt my eyes, he thought. Yet they are still good. In the evening I can look straight into it without getting the blackness. It has more force in the evening too. But in the morning it is painful. (2.31)
The old man’s body maintains its prowess because of his willingness to withstand pain.