Partially Submerged Castle is the eighth level in VIP1-WORLD1, as well the first castle of the game, in which Iggy resides. As suggested it is a partial aqua level.
This is a very straightforward. There're a lot of pits, but they're all flooded with water. The level uses mostly enemies to blocks your way, a couple of sections just having you dodge lots of Ball n' Chains. Towards the end of the level, you have to swim through a tunnel of more enemies. Then, the boss door will be before you, and you will face off against Iggy Koopa.
And so, after you defeat Iggy Koopa, Boon secures one of the seven Yoshi eggs, and the player is greeted with the next part of the story: