Dayshot: It was built by Planet Minecraft's Zeckou, who calls it a live coral reef, which explains the little floating fish and jellyfish. It's also deceptively big. Check below for the hi-res version of the three pics and the obligatory in-game shots, and click here for the world download.
As I said, it's pretty big:
Dayshot showcases some of the prettiest, funniest game-related screenshots and art that we can find.