How did Renny's cousin and friends react to the scratch marks on Renny's body and his fear? Why might they have reacted in this way?

The other boys didn't believe something hit him in the water; they thought he had been scraped by a branch. Renny ran back to town to tend to his wound, yelling at the other boys not to go back in the water. They didn't listen and jumped back in, again making noises and vibrations that came to the immediate attention of the shark. They reacted this way, because they could not believe that anything truly dangerous could be swimming in the Matawan Creek.