Here's why:
* Dialect: Twain is known for his use of dialect, particularly the language of the American South and the Mississippi River region. Putting "labboard" in quotation marks indicates that this is a word spoken with a specific regional accent or dialect, likely meaning "port side" (the left side of a boat).
* Hyperbole: Hyperbole is an exaggeration for effect. Quotation marks don't usually signify hyperbole.
* Sarcasm: Sarcasm is a form of irony used to convey a meaning opposite to the literal words. Quotation marks alone don't indicate sarcasm.
* Understatement: Understatement downplays something for effect. Quotation marks wouldn't typically signal understatement.
In summary: Twain uses quotation marks to highlight the unique pronunciation and vocabulary of a particular dialect.