The beach Jerry has always visited with his mother is very much associated with childhood innocence; even the narrator refers to it as "the safe beach." It is a place without danger where he is ultimately protected and watched over. When he looks back on it from the water, the description of his mother is telling:
There she was, a speck of yellow under an umbrella that looked like a slice of orange peel. He swam back to shore, relieved at being sure she was there, but all at once very lonely.
Sunny, cheerful colors of yellow and orange characterize that beach (and his mother). It is bright and secure: a place he knows well.
The "wild bay," on the other hand, is a place full of dangers. The water itself shows "stains of purple and darker blue," underwater "rocks lay like discoloured monsters" beneath the surface, and "irregular cold currents from the deep water shocked his limbs." This beach is full of hazards, the water perilous. The connotation of words like "stains" and "monsters" is overwhelmingly negative, while the image of purple and blue stains sounds like a bruise. Moreover, the shocking, cold currents are unpredictable and unpleasant as well. If the safe beach is symbolic of childhood, then this rocky beach and its dangers are symbolic of adulthood and the transition into it. Jerry is no longer protected as his mother cannot see him anymore. He is alone and wholly responsible for himself here.