How does James Reeves create a vivid representation of the ocean in the poem "In the Sea"?

In this poem, the sea is characterized as a "hungry dog," an extended metaphor which gives the poem its internal cohesion. The behaviors of the sea—its sounds and actions—are imagined vividly as a dog which "rolls on the beach all day" and "moans." Just as a dog might sometimes seem to be heard speaking, the sounds of the sea can take on a human quality: "Bones, bones!" The sound of the sea is equated to the moaning and yowling of a dog, depending on its mood.
The sea dog behaves differently according to the season. The image of the dog "shaking his wet sides over the cliffs" suggests vigor and power, creating a vivid mental picture of a wet dog shaking itself, scattering water. Meanwhile, "on quiet days," the sea, like a dog, can seem at rest, too, "so quiet" on "the sandy shores."
Through the use of this extended metaphor, the poet helps us to understand the sea as a living thing by comparing it to another creature with motivations and a personality of its own. The reader understands the sea to be both a powerful beast, and one that is capable of quiet.