Each line of this short poem hands something to the next, like a game of tag. The sea becomes a monster, the monster becomes the sky, the sky holds a key, and the key opens a door, then a chest, then the sea again. The chain loops back to where it began, so there's no real start or finish, just a circle of images that keep swapping roles.
The speaker seems more playful than frightened. Calling the sea and sky "monsters" could sound threatening, but the nursery-rhyme rhythm and the invitation to "play" keep things light. The last two lines shift to "we," pulling the reader in. To "play guilty" suggests pretending to take the blame, as if everyone shares a small, harmless secret.
The mood is dreamy and puzzling. Nothing gets explained, and the poem seems happy to leave it that way. The mystery isn't meant to be solved; it's meant to be shared.