While you explore the world of , you'll run into Issa Rock, a young girl with a thirst for knowledge. As you explore the islands, you'll find all sorts of side-quests, secrets, and more, but you can only find Hidden Reefs out in the open ocean.
These Reefs are found all around the seas of Concordia, so you have some work ahead of you before you can find them all. Note as well that some of the Hidden Reefs are only unlocked later on in the game, so if there are some you can't find, don't panic. Hidden Reefs are rocky structures that often look like enemies or animals.
To start this quest, talk to Issa on Shipshape Island, where she'll give you all the information you need to get going. After you talk to Issa, you can start your search in the Lushgreen Sea.
This location has three Reefs: the Weeping Lion Reef, the Pensive Reef, and the Squiddish Reef. They're all on the southern portion of this map and are relatively easy to find.
Note that some of the reefs that follow are only obtainable later in the game, so there's no shame in waiting until the end ofto return to your search.