A young woman slips out of her home at midnight and hurries through wet grass toward a lake, calling to a lover whose boat she hopes to stop. The poem saves its cruelty for the last stanza, where we learn that "Thy love's pallid corse the wild surges are laving". Her promise, "I come, dearest Henry, and wait but for thee," rings out while Henry is already dead. The central idea sits in the gap between her urgency and his fate. Tenderness and hope are intense and entirely real, yet they are powerless against what the poem calls "the stern voice of fate." The only comfort it offers is in the final lines, where the "parting spirit" is promised "a seat for thee" in "eternity's bowers." The poem never says how she dies, but that phrase suggests she does not survive him.
The construction is tidier than the storm it describes. Each of the three stanzas has eight lines and the same rhyme scheme, with the first four lines alternating and the middle three sharing one rhyme. Those middle triplets are double rhymes, such as "whortle, myrtle, kirtle," "soothing, moving, proving" and "laving, raving, saving." They pile up like a quickening pulse. Then the eighth line snaps back to the stressed rhyme of lines two and four. Read down those B-rhymes and you find a small story in the stanza endings: "I come," then "happiness flee," then a seat "there."
The rhythm is a galloping, anapaestic beat, which you can feel in "As she rapidly hastes to the green grove of myrtle." The line itself seems to run. This suits the girl's flight, but it also gives the poem a song-like lilt that sits oddly with grief. A modern reader may find the bounce a little too cheerful for a drowning. The music is that of a popular ballad, and it makes the sorrow feel formal and almost ceremonial.
The imagery sets small, gentle details against a hostile world. We see dew knocked from whortleberry leaves, a grove of myrtle (a plant traditionally linked with love), and a kirtle, or gown, pulled around her. Around her are a "stern" tempest, a "dreary" mountain and a "pitiless home." The poem never explains why the home is pitiless, so we are left to imagine the reason. The repeated "stern" is telling. The same word describes the storm in the first stanza and fate in the second, as if weather and destiny were one force.
The poem also moves from physical to emotional images and back again. In stanza two, affection "swelled in her bosom," and "sympathy's swell" moves her heart. By stanza three the "fierce swell of the tempest" is raving over Henry's body. Her feelings rise like water, and the water answers by killing. "Bosom" itself comes back three times, soft and loving at first, then "rend" by false hope. The word moves from tenderness to injury.
The speaker's position shifts as well. In stanza one the narrator watches in the present tense ("I see her swift foot," "I hear") like a witness who cannot intervene. Stanza two slips into the past, and stanza three breaks into the cry "Oh!", repeated three times, with two unanswered questions: "how could fond visions such softness deceive?" Finally the speaker turns to address the girl directly. Having failed to protect her, the narrator can only console.
As for significance, the poem is modest, and it helps to be honest about that. Shelley is one of the major English Romantic poets, known for work as ambitious as "Ozymandias" and "Ode to the West Wind." This poem is a different thing. Its stock ingredients are a midnight flight, a storm, doomed lovers and a consoling afterlife. Its phrases ("dear hope," "soft bosom," "fond visions") are conventional rather than daring, so the poem seems to work inside a familiar sentimental mode rather than against it. What makes it worth reading is how effectively it uses that mode. The dramatic irony of her hopeful promise, the rhymes that gather speed and the swelling water that echoes the swelling heart give a simple story real pull.