"under the sea" it relates to stanza 31, "Nor the demons down under the sea."
On stanza 3 is says, "That a maiden there lived whom you may know." This describes Assonance.
Sounds of the ocean waves.. on stanza 41 it says, "In her tomb by the sounding sea"
In the stanzas it says that even the angles were jealous but no one would know because angles don'e speak or show feelings to the living, On stanza 30 it says, "And neither the angels in heaven above." Angles can't feel anything.
In stanza 19 it says, "To shut her up in a sepulchre." Which means imagery because sepulchre.