To begin this quest, you need to complete the other four sections of the Watatsumi quest, then return to Sangonomiya Shrine and speak with Tsuyuko. When she knows you have the four pearls you will need to open the gate of the Heart of Watatsumi, she will direct you to the entrance.
Following the waypoint, then interact with the blue energy wall with the fish symbol on it to break the seal. Offer the four pearls and then head inside. Before you jump down inside, you want to assign numerical values to the four puzzles, as you need to complete them in order.
- 1 – directly in front of you
- 2 – to the left side
- 3 – to the right side
- 4 – behind the central dais
Move through each section in order. The first three are all the same. Turn the fish statues so that they face inward, then take out the Hydro Mimics that will spawn in and interact with the main pillar of the puzzle. When you get to the last one behind the central dais, interact with the pillar first, then align the fish.
Three waves of enemies will spawn in this time, so prepare for a bit of a battle. When you take out the last one, a cutscene will play showing Hydro energy gathering in the massive pearl at the top of the cavern. Grab the Key of the Deep and speak with Tasyuko again to finish the quest.