The main symbol in the poem "Cooks Brook" is the water to which the divers must plunge into. The boys, poised to take the plunge into the dark and dangerous water, consider the hidden challenges that lie just below the surface of the water.
Symbolically, the dive represents the leap into manhood for the boys. The boys know that
It would be better to die
Skull smashed open in the water
Than it would be to climb
Backwards down to the beach.
Therefore, the leap not only symbolizes the transfer to manhood (only the bravest prove themselves to be men by jumping), it also represents courage and strength--two characteristics looked highly upon when one is being considered a man or not.