Sidon will reveal a hidden treasure when you free the Zora kingdom from the muck. He will guide you through two waterfalls before telling you how to get there.
How to Complete the Secret Treasure Under the Great Fish in Zelda: Tears of the Kingdom
If Link does not remember to remove his Zora armor gear, he will automatically ascend the big waterfall beneath the city.
Once inside, the entrance to the Depths is a small hole. You can safely disregard it and launch your Paraglider nevertheless. The Vah Ruta Divine Helmet is hidden beneath the crashing waterfall.
This ancient item was often worn by the former Sage of Water. It will strengthen your connection to the Zora and give you the Swim Speed Up bonus.
