Jerry thinks of the beach where his mother goes, where he has always gone, as "the safe beach," and the other beach, where he longs to go by himself, as "the wild bay." At the wild bay, "rocks lay like discolored monsters under the surface" of the water and "irregular cold currents from the deep water shocked his limbs." It is dangerous and kind of ominous; the water is unpredictable and there is the possibility of injury, even death. It seems to represent maturity and adulthood, especially because that's where the older boys, who seem like "men, to Jerry" swim. On the safe beach, Jerry's mother sits, a "speck of yellow under an umbrella that looked like a slice of orange peel." This description seems to align the safe beach with immaturity and childhood. There, Jerry used to play under the watchful eye of his mother, who kept him safe, but at the wild bay, he is on his own with the "monsters" under the water.
The older boys swim through the tunnel at the wild bay, aligning it with maturity as well. It's as though that tunnel represents a rite of passage of sorts; Jerry seems to think that, if he can swim through it, he too will be a man. He behaves childishly, trying to get the boys' attention, and they eventually leave him alone. However, in the end, after Jerry has successfully gone through the tunnel, "He could see the local boys diving and playing half a mile away. He did not want them." Jerry seems to have grown up, a bit at least. Though he is still a bit childish, bragging to his mother about how long he can hold his breath underwater, "he [gives] in at once" when she instructs him not to swim anymore today. She'd expected a "battle of wills," but his maturity has increased, and this helps to frame the tunnel as a symbol of growing maturity or even a rite of passage to adulthood.