Water Land

Sea Side (alternatively Water Land, Ocean Side, Big Island, or the Island World ) is a water-themed world, and the third world in Super Mario Bros. 3. Sea Side has nine stages, two Hammer Bros., two fortresses, five Toad Houses, five Spade Panels, and a castle. Cheep Cheeps and Bloopers are common enemies in this world, and the hard-to-dodge Boss Bass also lives here.

The Koopaling that has taken over Sea Side is Wendy O. Koopa. The real king of the land (who bears a resemblance to Mario and Luigi) resides in a castle on an island chain that is in the shape of Japan, with the castle being located where the city of Kyoto would be, the city in which the Nintendo headquarters is located.

The map of Super Mario Advance 4: Super Mario Bros. 3 suggests that it is located to the south of Desert Hill, and west of Big Island.

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 Mushroom Houses and two Spade Panels and a second island with a third Mushroom House. It is possible to do the glitch in the Japanese version of the game, to do that, stay on the canoe and get a Warp Whistle. It will result in walking on water, and getting into a glitched map. It can also 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.

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, Bloopers and Jelectros.

Trivia

 * The music that plays in this land is very similar to the File Select music of The Legend of Zelda games.