The Solitary Sea Beast qyest is unlocked by exploring the Watatsumi Island in Genshin Impact. To unlock this quest, all you need to do is talk to Kumi (blue exclamation mark on the map) located on the beach near Watatsumi's Statue of Seven.
How to complete the Solitary Sea Beast Genshin Impact?
Once the dialogue has started with Kumi, she will explain to you that the bones next to her are those of her childhood friend, a sea creature. Your objective will then be to make some kind of offering to the creature.
Your first mission is to give 3 cherry blossoms to Kumi. You can find it everywhere in Inazuma and especially around the Sacred Cherry Tree.
Your second mission will be to go rescue Anisa against monsters.
How to solve the puzzle of the Watatsumi statue
The third step of this quest will require you to solve a puzzle. You will need to activate the three elemental pillars but a level 6 of Electrogranum is required. If your Electrogranum level is too low, you will not be able to complete this quest.
Once the three pillars are activated, the small statue will be released and you will be able to pay homage to it for the first time. This will then reveal the missing tiles. Go to the first slab to start your way. Stop where there is a slab missing, right next to the statue. Speak again to the statue and follow the path as shown in the video below to complete the quest.
Return to talk to Kumi to learn that the creature died of old age and has given birth to adorable other creatures. You will get the "A Mermaid's Tale" achievement and other rewards.