I swam across the skies last night, While stars floated on the sea. But when I tried to pluck a light, The stars all eluded me.
Then riding a cloud which was passing by, I attempted to capture the moon, But it flew back to its place on high, And the sea was but sea, too soon.
A dreamlike night turns the world upside down: the speaker swims across the sky while the stars float on the sea. It's a lovely, playful image, and for a moment everything seems within reach. But each time the speaker tries to grab something bright, it escapes. The stars dodge a reaching hand, and the moon, which the speaker tries to catch from the back of a passing cloud, simply flies back to its place on high.
The last line is the quiet sting. Once the moon is gone, "the sea was but sea, too soon," so the magic drains away almost as quickly as it arrived. The speaker doesn't seem angry, just a little wistful, like someone waking from a dream they wanted to keep. The mood is light and imaginative at first, then softly disappointed. The poem seems to say something familiar about wanting beautiful things: they glitter most when they're out of reach, and the grasping is what makes them vanish.