Water Land

Water Land  (alternatively Ocean Side, the Island World, Sea World, or Sea Side ) is a water-themed world and the third world in Super Mario Bros. 3. Water Land has nine stages, two Hammer Brothers, two fortresses, five Toad Houses, five Spade Panels, and a castle that has been taken over by an airship. Cheep-Cheeps and Bloobers are common enemies in this world, and the hard-to-dodge Boss Bass also lives here. This is the first world in Super Mario Bros. 3 to have more than one fortress.

The ruler of the swift tides of Water Land is Wendy O. Koopa. The real king of Water Land (who bears a resemblance to Mario and Luigi) resides in a castle on an island chain. The chain of islands is in the shape of Japan, with the castle being located where the city of would be, the city in which the Nintendo headquarters is located. During the events of the game, the Water Land king is turned into a kappa; the SNES and Game Boy Advance versions of Super Mario Bros. 3 see him transformed as a Dino Rhino.

The map of Super Mario Advance 4: Super Mario Bros. 3 shows that it is located to the south of Desert Land and west of Giant Land.

Canoe
If the player has a Hammer in their inventory, they can break a rock to gain access to the canoe. The player can then control the canoe across the water to an island with two Toad Houses and two Spade Panels and a second island with a third Toad House.

It is possible to do a glitch in the Family Computer version of the game; to perform, the player must stay on the canoe and get a Magic Whistle. It will result in walking on water, and getting into a glitched map. It can also be used to gain access to all the other worlds.

The Adventures of Super Mario Bros. 3
Water Land was only mentioned in one episode of The Adventures of Super Mario Bros. 3, and "The Ugly Mermaid" is the only episode where this world is shown as the title card.

Sneaky Lying Cheating Giant Ninja Koopas: Bully (though speaking through Kooky's mouth due to an animation error) suggested that they could take over Water Land after having conquered Giant Land.

Nintendo Comics System
Water Land is the setting of the Nintendo Comics System story Fins and Roses, which has a heavily armed Wendy O. Koopa set up an ambush for Mario, only to be annoyed by Stanley the Talking Fish while waiting for the plumber's arrival.

Nintendo Adventure Books
In Leaping Lizards, Morton and Iggy Koopa have both set up bases in Water Land, a filthy houseboat and an underground laboratory, respectively. After the Mushroom King is transformed into a rabbit by Morton and no one can figure out how to change him back, Luigi and Toad head to Water Land to find Morton and get answers, with some of the creatures they encounter being Cheep Cheeps, Bloobers and Jelectros.

Trivia

 * The music that plays in this land is very similar to the Fairy Fountain theme from The Legend of Zelda series.
 * The Nintendo Power strategy guide for Super Mario Advance 4: Super Mario Bros. 3 erroneously labels this world as "Big Island" when describing the boss.