This lyrical poem uses a straightforward ballad-type structure with ABAB rhyme in the first section, and then a shift marked by the line "Where billow meets billow, there soft be thy pillow," whose rhyme is entirely internal. This structure is echoed by the line, "The storm shall not wake thee, nor shark overtake thee." The structure of the poem has a sing-song quality which reflects its purpose: "Seal Lullaby" is intended to be the song of a mother-seal to her "weary wee flipperling," while the "slow-swinging seas" lull the baby to sleep. The rhythm of the poem echoes that of the waves.
The title, "Seal Lullaby," combined with the Scots edge in the phrase "weary wee flipperling" suggests that this poem may allude to the selkie legend commonly told in the British Isles, particularly Scotland, in which female selkies who live as seals in the sea are made human on land. They often mate with humans to produce selkie babies. In the sea, selkies are safe and free; on land, their husbands often steal their skins to keep them anchored there. There is a suggestion in this poem that some kind of metaphorical "night is behind us." The sea is "black," as if literal night still reigns while figurative night, some sort of danger, has now passed; the mother and baby have escaped to the welcoming "arms" of the sea.
The seal-mother in this poem reassures her child that only "the moon...looks downward to find" them; the baby may "curl at thy ease," safe from sharks and storms. The sea is not a place to be feared, but a true home, where the baby may "rest in the hollows that rustle between." The poet uses the metaphor of a pillow to emphasize the idea that the sea is a bed for the "flipperling," which may sleep easy in its "arms," the sea here personified as a guardian.
This poem is often set to music and performed as a lullaby.