It is one of those most English of poems, taking us to one of the many historic sites that helped define England as a nation. Arnold's poem takes advantage of Dover's symbolic significance and makes it a testing ground for more than just the sovereignty of a nation. In the personal sphere of the poem we are listening on one level to the anguished voice of inner conflict.
Faith appears to be under siege in the poem.
The sea that struggles and raves is the British Channel itself, flinging itself vainly against the land, an almost stock image in English Literature that has a number of possible symbolic connotations. For a different perspective on Arnold here is an essay about him that you might want to pursue.