The Fin of Watatsumi is a subquest for the Moon-Bathed Deep quest in Genshin Impact. Players will need to complete some fairly simple puzzles in order to progress past this quest.
Finding the Fin is simple, just open your quest list, then travel to the waypoint when you select the Fin of Watasumi quest under the Moon-Bathed Deep. When you arrive, read the tablet beside the Shrine, which will give you some basic information on what you need to do.
At the bottom of the slope behind you, you will notice two designs on the ground with some statues. Head to the one on the right, as you face down the slope. Turn all the fish statues to face inward, so the ground in front of them lights up with blue energy. Now, interact with the totem statue to complete that part of the puzzle.
Each time you move a piece on this site, the other side will also move, but the good news is you only need to make a single move on the left side to solve the puzzle. Interact with the fish statues shown above once, the complete puzzle will be solved, and the shrine seal will be broken.
Walk back up the slope to the shrine to interact with it and get the pearl that you need to progress the quest.