What is the main theme of the poem "The Sea" by J Reeves?

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 , 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."