From the Super Mario Wiki
The Floodgate 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 Torrent Seal, Cataract Seal, and Waterfall Basin Seal. The purple treasure chest containing it is in the lower left area of the ruins, in a room accessed by hitting a switch with Cosmic Wario's laser. The key is used to open a locked door with a picture of the key on it in the room to the left of where Wario first discovers the waterfall.