Jamie,

Twain served as a cub pilot on the "New Falls City" in 1858, according to Albert Bigelow Paine in "Mark Twain: A Biography," (chap 15), 1912, and in "The Boys Life of Mark Twain," (chap 27), 1916, same author. Paine gives his source as Horace Bixby, the pilot that Twain was serving under at the time.