Go to any Pianta Village episode where the water is in the spring and not in the tub for the Chain Chomp. One example would be the very first episode. Jump onto the tiny island with the old guy, surrounded by water. Then, position yourself behind him, by the tree. Slowly walk towards the water and Mario will be walking underwater. Oddly, you'll have no air meter, and still lose water when you shoot, and if you go under the bridge, and use Y or R to look around, the camera will sometimes get stuck at a very freaky angle.
Submitted by: Dyn721