Personification is a type of metaphor wherein human qualities are attributed to something that does not really have those qualities. In this poem, Reeves compares the sea not to a person, but to a dog. While this isn't strictly personification, it is a metaphor that attributes animal qualities to something else.
This poem is centered around the extended metaphor of the sea as a "hungry dog." Like a dog, the sea "rolls on the beach," with this imagery representing the sea's waves. The "moans" of the sea dog represent the sounds made by the waves, which at night become "howls and hollos." Each behavior of the sea is translated into a corresponding behavior in a dog, such as the way the spray of the stormy sea becomes a dog "shaking his wet sides over the cliffs."
Like a dog, the sea can also be calm. The final stanza depicts the sea-dog "so quiet, so quiet" as he lies on the shore, representing the sea at rest in the quiet days of summer.