Find ten examples of imagery that pertain to SHAPES AND COLORS.
I am not going to give you 10 - I will give you a few and then you can use them as examples to re-read the story and find some more. Obviously what you want to do after you have found these examples is to consider how they relate and add to the story as a whole. One aspect you will want to consider is how the two beaches - where Jerry's mother is and the other beach where the tunnel is - are compared, and the kind of imagery that is used to compare them. Consider this quote describing the beach of the tunnel:
From where he was, high up among red-brown rocks, it was a scoop of moving bluish green fringed with white. As he went lower, he saw that it 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 years, he saw an edge of white surf and the shallow, luminous movement of water over white sand and, beyond that, a solid, heavy blue.
And what about this one describing the "safe" beach of his mother:
There she was, a speck of yellow under an umbrella that looked like a slice of orange peel.
Clearly imagery is used to make the "dangerous" beach appear exciting, mysterious and scary, whilst more domesticated images are used to present the "safe" beach. That should get you started.