World 4 (New Super Mario Bros. Wii)
From the Super Mario Wiki, the Mario encyclopedia
World 4 (also known as the Water World) is the fourth world in New Super Mario Bros. Wii. It features several tropical islands, therefore, most of its levels are water-themed and make rather good use of the Penguin Suit, which can glide over water and allow Mario to swim better. Also, on the map is a bridge guarded by a Porcupuffer that can jump up and make Mario and the gang endure an Enemy Course. This is also the first world to have a level on an airship, like the ones in Super Mario Bros. 3. Wendy O. Koopa is the boss of this world. It has nine levels and a total of eleven exits. One lone island appears, though it serves no significance.
World 4 also appears in Mario & Sonic at the London 2012 Olympic Games under the name "Cheep Cheep River" as the location of the Dream Rafting event. Porcupuffers, Thwomps, Cheep Cheeps, and Bob-ombs appear in it, as well as the addition of Gooper Blooper. Also, Wendy (the boss of this world) appears as a costume for the player's Mii.