The Floodgate Key is an item found within Oracle of Seasons.
When Link first arrives within Spool Swamp, the floodgates are closed, preventing Link from accessing the southern portion of the region. Link can enter the Floodgate Keeper's House where he will find the Floodgate Keeper. However, the man confesses that he has lost the Floodgate Key.
Link can hit the lever in the corner of the house to lower the water, revealing the location of the Floodgate Key. This key enables Link to open the floodgates so that he can reach the southern portion of Spool Swamp. This area will eventually lead to Link entering the third dungeon, Poison Moth's Lair.