What contrast exists between the beach and the bay?

To begin, the beach is identified as "the safe beach"; it is the beach Jerry and his mother have always visited on holiday.  It is a place that he associates with his childhood, with safety, and with her protection. 

The bay, on the other hand, is "the wild bay," and, as Jerry drew nearer to it,

he saw that spread among small promontories and inlets of rough, sharp rock, and the crisping, lapping surface showed...

To begin, the beach is identified as "the safe beach"; it is the beach Jerry and his mother have always visited on holiday.  It is a place that he associates with his childhood, with safety, and with her protection. 

The bay, on the other hand, is "the wild bay," and, as Jerry drew nearer to it,

he saw that spread among small promontories and inlets of rough, sharp rock, and the crisping, lapping surface showed stains of purple and darker blue.  Finally as he ran sliding and scraping down the last few yards, he saw an edge of white surf and the shallow, luminous movement of water over white sand, and, beyond that, a solid, heavy blue.

Many of the word choices here carry a dangerous connotation: rough, sharp, stains of purple and darker blue (which sound like bruises!), scraping, edge, and heavy.  Many sound like words we associate with weapons or the injuries caused by them.  They all seem painful and damaging.  Further "rocks lay like discoloured monsters under the surface, and [...] irregular cold currents from the deep water shocked his limbs."  The mood associated with the wild bay is therefore very ominous: there are "monsters" under the water and cold currents to "shock" Jerry.  This sounds very unpleasant.

However, when he looks back at the safe beach, he sees his mother.  "There she was, a speck of yellow under and umbrella that looked like a slice of orange peel."  Instead of the dangerous and ominous imagery and word choices associated with the wild bay, the safe beach is characterized by citrus colors, colors we might normally associate with a carefree beach vacation.  It is very much a safe place compared to the dangers of the wild bay.