As you progress through Hogwarts Legacy, there are a slew of sidequests for you to complete. Some of these sidequests may be from the residents of the surrounding burrows, but most of them will be from your fellow classmates. Nerida Roberts is a natural swimmer, but she wants nothing more than to receive a gift from the merpeople. Follow this link to learn how to complete 'Mer-ky' Depths in Hogwarts Legacy.
Walkthrough for Hogwarts Legacy: 'Mer-ky' Depths
Nerida can be found sitting on the edge overlooking the water once you have completed the first of the trials. This is where the merpeople have left her in a secret place called the Leaky Cave.
After receiving the quest, jump off the dock and swim toward the cliffs around the castle. You will notice a whirlpool-like area in the water and you will swim down into it when you emerge in the Leaky Cave.
How to Find and Get Beasts in Hogwarts Legacy
In the Like Moths to a Frame side quest, you must walk around the cave and use Lumos to attract moths. Once all three moths have landed on the door, the door will open. Interact with the chest to obtain the item for Nerida.