A hidden puzzle is inserted in underwater Fontaine where you need to save 7 different otters to get treasure chests. Once all 7 otters are rescued, you will get the last Precious Chest as thanks.
All Otters Location in Genshin Impact Fontaine
All of the otters are in underwater Fontaine albeit very far from each other. Make sure to go to locations 1-7 first and save the 8th for last because the Precious Chest there will only appear once you rescue all 7 otters.
A little reminder that if you accidentally hit the otter with any ability (before the treasure chest is spawned), it will run away. But you can re-login to spawn the otter back and it will return to lead to the chest.
➥ Teleport to Annapausis south waypoint and head south towards the barrier that divides the underwater area and Annapausis. Swim upwards towards the debris and you will see an otter near a crystal and a Blubberbeast. Absorb the Blubberbeast’s ability and use it to hit the crystal (there’s an invisible barrier). You can also go to the opening at the side of the building to attack the crystal directly but I find the Blubberbeast ability much more useful and easier.
➥ After the otter wakes up, follow it until a Common Chest is spawned and you can take it. Do this for every other otter to get the chests and count the rescue mission as successful.
➥ Teleport to Chemin de L’Espoir southeast waypoint and swim southeast towards a Fantastical Floating Ball puzzle. Near this puzzle is a Bullet Barnacle. You can find an Armored Crab behind the wrecked building so absorb its ability and use it to destroy the Bullet Barnacle and free the otter behind it.
➥ From the previous location, swim northeast towards another buried building and you will see an otter being cornered by a Clockwork Meka. Defeat the Meka with any ability you have to rescue the otter.
➥ Teleport to Elton Trench waypoint (right above the writing) and swim east towards the edge of the underwater map. The otter in this one is also trapped within a Bullet Barnacle so destroy it using Armored Crab’s ability (which conveniently can be found near the Bullet Barnacle).
➥ Teleport to Salacia Plain waypoint (just beside the writing) and swim northwest towards the side of the aquabus center. One of the otters is locked inside the cage, and you just need to open the door, no need for any key.
➥ Teleport to Thalatta Submarine Canyon waypoint (right below the writing) and swim southeast to an otter trapped inside a Seagrass. But before that, you will pass through a Hunter’s Ray so take their ability because you’ll need it for this one. There are 2 weak links on this Seagrass, and you need to cut both of them at the same time or it will respawn back.
➥ From the previous location, swim further southeast towards a building with lots of Blubberbeasts in front of it. The otter is inside the box and you can just use the [Left-Click Mouse] button to destroy the box without accidentally hitting the otter inside it.
➥ Now that you saved all 7 otters, from the previous location, swim again toward the southeast and you will get an icon notification about a treasure chest. This chest will only spawn after you save all 7 otters (There’s no hidden achievement for this puzzle).