Final answer:
The most fitting mood descriptor for the passage from 'The Adventures of Huckleberry Finn' is eerie, due to the spooky imagery and the heightened tension experienced by the protagonist.
Step-by-step explanation:
The word that best describes the mood in the passage from The Adventures of Huckleberry Finn by Mark Twain is eerie. The excerpt depicts a tense and frantic scene as the protagonist navigates through dangerous waters, encountering 'smoky ghosts of big trees' and barely avoiding 'snags that fairly roared'. The use of vivid details and imagery, such as the protagonist listening to his heart thump, creates a heightened sense of anxiety and eeriness, encapsulating the reader in the spooky and ominous atmosphere that Twain is renowned for.