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.

Reeves compares the many moods of the sea to a "hungry dog." In doing so, he illustrates that the sea is angry and unpredictable. During the day, the dog "rolls on the beach all day/ With his clashing teeth and shaggy jaws" and at night, he "bounds to his feet and snuffs and sniffs/ Shaking his wet sides over the cliffs/ And howls and hollos long and loud."

Reeves ends the poem by telling us that even the "hungry dog" from above has a quiet side and can be peaceful and serene. In this way, we can compare the sea and the dog that represents the sea to humanity and its many moods.

One element of the poem that really strikes me is its mood.  We commonly read poems that depict nature, even the sea itself, as tranquil and calming, or stories (especially movies) about dogs being "man's best friend."  But this poem does something different with both.  The idea that the sea has "clashing teeth and shaggy jaws," that it "gnaws" and "roars" like an angry, hungry animal, is a strikingly original comparison: a metaphor, to be exact, as other commenters have pointed out.  Therefore, the mood is not peaceful, and it is not friendly; instead, it is off-putting, concerning, even frightening.  The unexpected mood draws attention, then, to the idea that even things that seem lovely, things that are most often depicted as beautiful or kind, can still have the power to be destructive and menacing.  Even when the sea is tranquil or the dog is companionable, when either one "lies on the sandy shores," there is still a beast capable of doing real damage underneath that temporary "quiet."

"The Sea" by James Reeves is rich in imagery and in creative metaphorical relations made between the sea and the idea of a dog. The poem is not, however, rich in theme. In discussing the formal theme of the poem, we can simply point to central comparison made in the poem that identifies the sea as being akin to a dog. 

The sea is a hungry dog, 
Giant and grey.
He rolls on the beach all day.

Formally speaking, the theme of "The Sea" is highly focused on the equivalency between the sea and the idea of a dog. The sea experiences joy and feels forlorn. It is happy and it is lazy, etc. 

To look at the intellectual or semantic themes of the poem, we might assess the poem's comment on the sea as an emotional being. The sea, as a representative of nature (or the natural world), is a living thing with passions and joys and moods. 

And when the night wind roars
[...]
He bounds to his feet and snuffs and sniffs, 
[...]
And howls and hollos long and loud.

With this personification of nature, the poem may be suggesting that the world around us can be engaged in some ways as if it were alive, awake and aware. We might consider the whims of nature and its appetites as elements to be feared and appreciated and recognize that creatures with appetites can be unpredictable.  

We might also read the poem as implying that the sea is only a small part of a larger natural system and therefore is subject to forces beyond its control just as we are -- or just as a dog might be. The sea experiences the weather and the sunshine and reacts. It may be huge and powerful but the sea is not in command of hot and cold, of rain or sunshine. 

These readings are available in the poem, yet the overall sense the poem projects is one of camaraderie and understanding. The voice of the poem is less awed by the sea than it is sympathetic with the playfulness and lonesomeness of the sea. 

This poem, which is an extended metaphor, derives its meaning from this controlling metaphor, that of the sea being like a dog.  While not a particularly complex poem, "The Sea" by J. Reeves does make use of a fresh comparison with the sea and a dog; in addition, the imagery is imaginative and alive.

The second stanza particularly has strong auditory imagery and visual imagery.  For instance, the "night wind roars" and the "moon rocks."  The dog imagery fits well to the comparison to the waves that come upon the shores at night:

Shaking his wet sides over the cliffs,
And howls and hollos long and loud.

The alliteration also moves the lines forward, like waves.