I am not aware of the story 'The Sand Reckoner'
Therefore, the question 'How do Marcus's words about needing to disembark contribute to the plot of The Sand Reckoner', means that there was a boat in the story of something similar to it.
Therefore, the character Marcus says that the other characters have to leave (disembark) the boat or the something similar to it.
Therefore how does this seem to change or help the plot in the story.
I hope this helped - Maria S.
