Why didn't James Reeves didn't compare the sea to a well fed dog?
It wouldn't have the same effect. By comparing the sea to a
hungry dog, Reeves employed a sense of fear and desperation in the
reader. The 'hungry dog' metaphor also allows the reader to compare
the waves of the sea to 'eating' the coast.
Techniquically, the subject matter of the poem is a storm at
sea, therefore wouldn't fit the metaphor of a well-fed dog.