"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.