A sealed Precious Chest locked a key to obtain Hydroculus floating high in the air. Solve the Seelie puzzle first to unlock the seal and obtain the Hydroculus.
➥ Teleport to the Court of Fontaine northwest waypoint and head further west to the shore until you see a locked Precious Chest. For this puzzle, you need to find 3 Seelies in this area.
➥ The first Seelie is inside the cave blocked by a breakable stone (Use Elemental Sight to spot the stone). There are Water Phantasm with a locked Exquisite Chest near the breakable stone, but it has nothing to do with the puzzle so you can defeat/ignore them if you want.
➥ For the second Seelie, you can use the Echoing Conch near the sleeping Blubberbeast (they’re not hostile until you attack them). The conch will mark the spots where you can dig to get something. The correct spot is southeast of the Precious Chest near another Blubberbeast that was also resting.
➥ The last Seelie is floating in the air and you need to use the bubble teleport south from the Precious Chest to get on top of a waterplank (also need to be dug out first from the sand).
➥ Once all 3 Seelies have returned to their home, a wind current will be activated and you can glide above to get the Hydroculus floating in the air.