The Floodgate Key is a quest item from The Legend of Zelda: Oracle of Seasons. If Link flips the switch inside the Floodgate Keeper's house, the pool of water directly above it will dry up, revealing the Floodgate Key. Upon exiting the house, the pool of water to the right of the house will be dried up, revealing a staircase. After going through the cave, Link will appear directly left of the Poison Moth's Lair. When he uses the Floodgate Key on the Floodgate lock to the southeast, the floodgate will open.
After all the water flows out, the water will be shallow enough for Link to walk through and he will now be able to continue south. Depending on which animal companion Link chose, he will be able to navigate one of three paths using that companion. After navigating a path, Link will find another portal to Subrosia, where he can have his Rod of Seasons imbued with the power of Summer.