My explanations for quotes on page 21

"I'm ready now," the old man said. " I only needed time to wash."
I think this quote seems almost self explanatory. I think that this line was intended to foreshadow upcoming events in the novel or in Hemingway's life. More specifically the fact that this book was written a few year before his suicide (if we're saying this is his suicide note) can indicate that it was done so to explain that Hemingway wanted to get things in order or deal with unfinished business. ie. Write a great novel.

"They lost today," the boy told him.
"That means nothing, The great DiMaggio is himself again."

I think this is another metaphor but this time it's about Hemingway's own career. (I don't know much about his career though) Maybe he's saying that he hasn't written a great book in a while and that's DiMaggio losing but now that he's got his inspiration or motivation, he's back and ready to be a winner, like DiMaggio.