The poem, in a simple sense, is about the speaker's trip at night to meet his lover somewhere out there in the dark.  Browning begins by creating a stark contrast between the speaker and his hyped up state and the very dark and quiet, immutable landscape.

As the speaker gets closer and closer to shore, he gets more and more excited, in some ways his behavior mimicking the waves which he is traveling through.

The ending comes only after he traverses several fields and a long, sandy path but then the darkness is broken, literally and figuratively as he is united with his lover and she strikes a match to light up their meeting.