Jerry’s insistence that he needs a pair of goggles relates to his inability to replicate the underwater stunt which the foreign boys he had encountered on the next beach over have been doing. After diving into the water, the boys are reappearing “on the other side of a dark rock”. The rock, which appears to Jerry to be an impenetrable wall, clearly has “some gap or hole” through which the boys are passing, but Jerry simply can’t find it. His problem is partly that he is presumably less accustomed to swimming in salt water than the other boys are. The salt water is stinging his eyes to the point where he cannot see anymore, and this is why he insists to his mother that he needs a pair of goggles.
Jerry’s reason for needing to be able to keep up with these other boys runs deeper than typical children’s competitiveness. It seems that since his father’s death, it has been just Jerry and his mother trying to carry on, both being as kind and accommodating to each other as possible. Having gone to a different beach from his mother, he does not feel the usual pressure to consider her feelings, and is therefore free to indulge the natural sense of competition that he feels when trying to keep up with these other boys. Having failed to keep up with them that day, Jerry hopes that a pair of goggles will change his fortunes the following day.