The Submerged Castle is the hardest dungeon in the Perplexing Pool and is most likely the last dungeon you will go to there. The boss of this dungeon, the Waterwraith, appears on all 5 sub levels after 5 minutes has passed while exploring each sub level. You can only bring Blue Pikmin in as the dungeon is located in the water. The Waterwraith's weakness is Purple Pikmin so attacking with the Blue Pikmin, White Pikmin, and Bulbmin is futile. There is no way to harm the Waterwraith before sub level 5, as the Violet Candypop Buds are located on that sub level.