The traders of Hyrule, they have been responsible for maintaining the water passageways. They house their cog in the Lake Hylia Temple ,their most sacred grounds. They are stuck in a conflict with the River Zolas, who have kidnapped the heirs and invaded Lake Hylia
Heading back to Hyrule Town after finishing up in the Deku Swamp, Link goes to meet with the Gerudo King, who tells Link he's discovered that the rest of the cogs are with the Zora, Gorons, and the king says that the final one is kept by the Hylian royal family.
Link, after the Zora and Goron cogs, heads out of the castle, but is caught by guards on the way out. Before he is thrown in the dungeon, however, Zelda happens by and recognizes him. She has Link sent out of the castle instead of imprisoned, but guards now block the path Link has been using the get into the castle/visit Ganon.
On his way out of the town, Link happens across a Deku Merchant, whose goods were stolen by a band of traveling Zola. In the field, Link finds the Zola and fights them. He loses, but the Zola abandon the fight and goods when they hear that, "hostages have been taken."
Returning the Deku Merchant's goods, he will lower the price of his flippers, allowing Link to buy them and access the river valley of the Zora. In the Zora's town, Link finds the everyone panicked, as their leaders have been kidnapped by the Zola and taken to forts along the river.
Either sneaking or battling his way into the forts, Link rescues the prince, princess, and King Zora. The Zola flee their forts after their defeat, opening the path into Lake Hylia.
At Lake Hylia, Link meets a man named Sheldon, who is studying the ancient mechanisms that dot the lake shore. Upon investigating one, Link is attacked by Maple, taking a more active role now in deterring Link. She is bested and flies off, but during the fight the mechanism was broken.
Sheldon sets about repairing it, while Link goes to find the rest. He activates each one, and returning to Sheldon, they activate the last one, which raises the entrance of the ancient Lakebed temple. Link approaches it, and overhears Maple talking with the Hero's shade. She goes into the temple, and the shade speaks to Link, warning him to avoid tampering with things he doesn't understand or he could bring the lives of everyone in Hyrule to an end.
Regardless of the hero's warning, Link enters the temple, claiming the water cog and advancing his quest.
On his way out of the river valley, Link is attacked by the remnant of the Zola, wanting revenge for their defeat. However, before Link can fight, the prince and princess of the Zora arrive and defeat the Zola. They share with Link that his courage inspired them to become stronger and take action to decide the course of their lives. They give him the Zora seal, an object signifying the friendship of the Zora people. This item will evoke different reactions from Zora characters Link meets now.
Link heads back toward Hyrule town...