Why do you think Santiago starts to regret hooking the fish at the end of The Old Man and the Sea?
Santiago began to wonder if he wouldn't have been better off not hooking the fish. It has cost him his knife, some of the other tools of his trade that he depends on and he is worried that he will not be able to fish once he returns.
He also is beginning to wonder whether this fish itself should have been left to live as it was so powerful and so majestic and won't be appreciated alive or dead because it will only be a partial skeleton by the time he returns to shore. The fish was noble and powerful and beautiful, but it is destroyed by the sharks and the weather and the very long time it takes to return to shore.