While the story of Noah's Ark, and the great flood are obviously a fairy tales, the quote is a great one. We can know that the idea of a global flood is a myth is from geological evidence.
Mark Twain is obviously famous for writing The Adventures of Tom Sawyer and The Adventures of Huckleberry Finn. The former is rather light-hearted, but the latter addresses some important contemporaneous themes, most importantly racism, which was rife in the southerns US states. From a religious perspective, Twain could be be described as a deist, at least that is what I can gather from Wikipedia.