The Submerged Castle has the Waterwraith, one of the most dreaded and feared enemies in the series. It is a dungeon in the Perplexing in both Pikmin 2, and Pikmin: The Pollution.
Pikmin: The Pollution
It has 5 floors, with the Waterwraith battled in the 5th one, and will drop from the ceiling on the other floors, after 3 minutes.
Smoky Progg - Floors 1,2,3,4
BOSS Waterwraith - Floors 1,2,3,4,5
4 De-Pollutionater Parts - Laying around on Floors 1 through 4
De-Pollutionater Part - Dropped by Waterwraith
30 Blue - Waterwraith
You can only have Blue and Green pikmin in here.
The easiest way through is by letting the Waterwraith on each floor kill green Pikmin. The Boss Waterwraith, like all other bosses, are unaffected, so make them purple ones with the candypop bud on its floor.
Another strategy is to quickly have one captain get the exit ready, while the other gets the De-Pollutionater part.
When the Waterwraith appears, have one captain bait it, while the other works.
Get both captains on the part, then get to the exit.