From the Super Mario Wiki
Deepwater Dungeon, otherwise known as World Bowser-5 is the fifth level in World Bowser from Super Mario 3D World, becoming accessible along with Footlight Lane after the completion of The Bowser Express. This level takes place in a dungeon with moving bodies of water, Spike Blocks, and a short Plessie section. Its completion unlocks A Beam in the Dark.
The level starts off with the player falling down before sliding down to a pit of water that rises/drains every few seconds that has Madpoles. After that, the player will have to swim on a series of moving bodies of water before reaching the checkpoint. On the second body of water, the player must avoid blue Spike Blocks. This level's stamp can be found in this area.
The next area includes players riding on Plessie before a Donut Block bridge and a body of water with red Spike Blocks. Then the player traverses a series of moving bodies of water while avoiding moving spike walls. After that, the player can go through a Warp Pipe or use a Cloud Cannon to head to the Goal Pole. There is a 1-Up Mushroom behind the Goal Pole.
In the first set of moving bodies of water where the player must avoid blue spiked walls, the player should go up the top left path where the stamp is in an alcove.
Pixel Luigi Sighting
Near Green Star #2, the player has to climb up to the sidling on the right and move southward. A pixel Luigi will be behind the stone slab at the end.
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