From the Super Mario Wiki
The Cataract Seal is a key found in Wario: Master of Disguise. It is one of four keys found in the Ancient Waterworks, the others being the Floodgate Seal, Torrent Seal, and Waterfall Basin Seal. The purple treasure chest containing it is in a room on the third-highest floor of the ruins, which has a gate operated by a switch. The chest is accessed by using Cosmic Wario's reflecting laser to hit the switch from the right side of the gate. The key is used to open a locked door with a picture of the key on it in the central area with the waterfall.