To start the Secret Treasure Under the Great Fish side quest in The Legend of Zelda: Tears of the Kingdom, speak with Sidon in the throne room at Zora’s Domain. For this quest, head to the center of the Great Zora Bridge.
Once there, glide into the waterfall to discover the Cave Under Zora’s Domain. Once in this cave, head behind the waterfall to find the chest containing the Vah Ruta Divine Helm. Once you loot the chest, the quest will complete.