Arguably, the main theme in the poem "The Sea" is the power and playfulness of nature, represented here by the sea.
The sea is described, metaphorically, as "a hungry dog" that "rolls on the beach all day." The implication here is that the sea is alive, and when its waves crash against the beaches and roll back with the tide, it is merely playing. In this way, the sea is presented as carefree and playful.
There is also some suggestion in the second stanza of the poem that the sea can also, like a dog, be menacing as well as playful. For example, the dog, as a metaphorical representation of the sea, is described as having "clashing teeth." It "gnaws" the stones, "moans," and "howls and hollos long and loud." Here then the poet implies the wild side of the dog, and, in turn, the wild side of the sea.
In the third and final stanza, the dog is described as resting with "his head between his paws." The implication is that he, and thus the sea, is tired after its playing. This compounds the impression that the sea is a living organism which has spurts of energy and different moods, just like a dog.