Jerry wasn't crazy, but he was very foolish to take such risks. He easily could have drowned in the dark tunnel, and it is reasonable to assume that his body might never have been found. His young life would have been lost, and his mother, who loved him and tried hard to keep him safe, would never have known what had happened to him. One can only imagine her heartbreak and continuing sense of loss and confusion. She might have spent the rest of her life looking for him as a lost child.
The fact that he would risk his life so dangerously suggests that he was motivated by a deep need. Jerry was growing up and beginning to feel the need for independence from his mother. Also, he felt humiliated by the older boys on the wild beach. He wanted to be as strong, fearless, and masterful as they seemed to be. He made a plan and went into training, secretly, to prepare for his swim through the tunnel.
Jerry almost died in the tunnel, but he had been strong enough to survive. He was shaken by the ordeal, but he was no longer a little boy. After he had accomplished what he set out to do, he told no one. He no longer "needed" the boys at the beach. He did not need their approval or acceptance because he had come to approve of and accept himself. He had gained self-respect.