Symbolism, Imagery, Allegory
Remember all that stuff about acknowledging the hopeless void but turning around and choosing to live life anyway? You could argue that the one thousand dollar check is a symbol for the fact that Grace chooses to live her life anyway. In choosing to accept the check, she is taking advantage of an opportunity that is presented to her, rather than turning away from the figurative promise that one thousand dollars offers her.
Also, you might think about what else that check represents. Why did Mr. and Mrs. Travers decide to give Grace a one thousand dollar check? What does that money allow Grace to do? Thinking about these questions will probably be a much more cheerful endeavor than thinking about that endless body of cold flat water.