Yona has been entrusted with the task of repairing your Zora Armor, and just requires one more component.
After you start the quest Sidon of the Zora, go slightly southwest from the statue of you and Sidon, then go up one of the large curved stairs. It leads up to the area with the throne room. Instead of going into the throne room, go to the area behind it, where you will find Yona. Talk to her to start this quest.
Talk to Dento
Go back around to the southwest side of this area, then go down one of the large curved stairs. Then go northeast and enter the item shop, which is on the right. Talk to Dento in the back. He will tell you that the Ancient Arowana has been seen near the statue of Mipha on Ploymus Mountain.
Reach the Skyview Tower
To reach the statue, start by visiting the Upland Zorana Skyview Tower. One way to get there is to cross the northwest bridge of Zora's Domain, then follow the path southwest. It leads to Ralis Pond, where you can gather many materials. Then go northwest from the pond to find the entrance to the cave Upland Zorana Byroad. This cave leads you to the Climbing Boots, and gives you an easy way to reach Upland Zorana Skyview Tower.
After you throw a Splash Fruit at the tower entrance, use the tower to map the area. Then skydive to Prince Sidon's location, which is marked on the map.
Get an Ancient Arowana
After you talk to Sidon, you should go southwest to the statue of Mipha. You will find some Ancient Arowana swimming around the statue. Catch one.
Talk to Yona
Now that you have an Ancient Arowana, go back to Zora's Domain and talk to Yona in the infirmary, which is beneath the throne room. You will receive the Zora Armor, which will allow you to swim up waterfalls. This completes the quest.