In here, use Captain Wario to swim against the current and reach the left 
ledge. Head into the next room by using the final key in your inventory. In 
the large waterfall room, go to the seesaw and stand on the right end. Use 
Arty Wario to drop a box on the left end to catapult yourself (switch to 
Thief Wario as soon as you make the box). On the ledge with the big door, use 
Arty Wario to stack two boxes to form steps leading to the right. Jump from 
them to the high right ledge, where you will find a chest containing the 
"Fanny Armor".