In the first stanza of the poem, the imagery of the "grey sea and the long black land" is rather dark, and the idea of the "startled little waves that leap / In fiery ringlets from their sleep" is not a particularly positive one (lines 1, 3-4).  Gray and black do not have light-hearted or hopeful connotations; they are often seen as rather sad, even morbid.  Then, the waves are personified as being startled (usually not a good thing).  This stanza is characterized less by a tone of excitement and more by the speaker's sorrow over being at such a great distance from his love.  Things seem to be moving slowly.

However, after the break, in the second stanza, the pace picks up and the speaker uses imagery that is much more pleasing with a much more positive connotation: the "warm sea-scented beach" and "two hearts beating each to each."  The narrator's sense of anticipation and excitement seems conveyed by the more precise distances he must travel in order to reach his lover.  It is no longer "the long black land" but rather "three fields to cross"; it is almost like a countdown!  Then, the excitement of knocking at her window and whispering so as not to be caught also represents a change in tone.  This stanza is a great deal more exciting and hopeful as the narrator finally gets within figurative and then literal reach of the person he loves.