The U.S. was the dominant whaling country at the time of Moby Dick, but there is little in the book that assumes a knowledge of American history, culture, or literature. It's a candidate for "the great American novel" only because it was written by an American. I've read it more than once, and it strikes me as something completely original, not tied to the conventions or themes of literature of its time or country of origin. You may or may not like it, but "Americanness" won't be the reason.