The legend of the ocean is one of the petitions in Pokémon Legends: Arceus.
In the Petition 66 Professor Lavender will ask you to find an ocean Pokémon linked to a mysterious legend.
This is one of the longest missions with the most stages in the whole game (you’ll have to carry several Pokémon along the way), but don’t worry, we’ll tell you how to complete all the stages of the game. mission to accomplish the reward of La Leyenda del Océano: Manaphy. (You deserve the pain!)
On this page you will find:
How to Unlock Petition 66 “The Legend of the Ocean” in Pokémon Legends: Arceus
Petition 66 will appear on the board as you progress through the game’s main story. It’s okay to show up after the credits, as it’s one of the late hours. When you have accepted the painting, Professor Lavender will speak to you.

Lavender will show you that he saw a mysterious Pokémon in Playa del Albor. I believe the ocean currents should have swept the Costa Cobalto away.
At that time, I will tell you that for a long time Costa Cobalto only called the East Sea and hosted a special Pokemon. Without hesitation, his name is written in a book whose whereabouts no one knows.

This reference links Pokémon Legends: Arceus to Pokémon Brilliant Diamonds and Beautiful Pearls, so the book you are looking for is not in this game but in the Sinnoh remakes. You can see it in this video:
Knowing this, how do we follow?
How to get the Pokemon you need for ‘La Leyenda del Océano’
According to the legend that can be found in Pokémon Brilliant Diamond and Beautiful Pearl, you will need a Buizel, a Mantyke and an Overqwil to complete this petition.
Buizel appears in Pradera Obsidiana, specifically in Prado Herradura, Vereda del Viento and Puente Erosionado. You will also find large-scale creations in Costa Cobalto, specifically in Cerro Aipom and near Albuferas Chapuzón.
They appear at any time of the day or night, so you don’t have to leave too much trouble.

Mantyke appears on Costa Cobalto, specifically in Ginkgo Beach and Silent Bay.

They are neither overly aggressive nor shy, but they sometimes disappear under the wings when they die. The best time to catch them is when you’re jumping out of the water, and if you’re also jumping with Basculegion to calm down, you can safely aim.

Overqwil is one of the new Pokémon in Pokémon Legends: Arceus. Throughout the story, you’ll see an alpha, but if you don’t capture it, you can evolve into Hisui’s Qwilfish.

Qwilfish appears on the Costa Cobalto, specifically in Bahía Silente, Guarida del Gran Pez and Costa Ínsola. You can also see them at Pradera Obsidiana, in the rivers near Isla Hansa. They’ll spawn at any time of the day, but they’re aggressive, so you might have to fight to get them.

Make sure Qwilfish has the Thousand Toxic Movement and dominates. You can take it up a notch, but if that fails, even at Villa Jubileo Dojo.

Once Hisui’s Qwilfish dominates Mil Puas Tóxicas, you’ll need to use it with Strong Style 20 times. Then you can evolve Overqwil from your Zurron.
How to complete Petition 66 “The Legend of the Ocean” and bring it to Manaphy
Once you have Buizel, Mantyke, and Overqwil, put them in your active squad and head to Costa Cobalto. You will wait later. Look at the “finger” between the arena hand and see two huge rocks sticking out of the ocean and forming a sort of doorway.

In the afternoon, as we said, we went to Basculegion and passed through this gate. If done correctly, there will be a brief dialog indicating that you heard a question in the distance.

This means that the Cabo Rocavelo cave has opened up, to the north of the map.

Enter the cave and you will find Manaphy as well as several Phione. Typically, they’ll circle around, so you’ll need to hit twice to start the fight. It will be a 4 against 1: Phione is level 35 and Manaphy is level 50.

The priority is to capture Manaphy so I can teach Professor Lavender. If all of your Pokémon lose consciousness, you can return with new Pokémon. Some of the lost Phiones will be restored.
Pokémon Legends: Arceus has arrived in stores! We have a guide with tips and tricks to prepare, in addition to help like the complete Hisui Pokédex and the new Pokémon Hisui.
In addition, we help you choose between Hisui’s initial Pokémon.
We also help you with new techniques -Crafting, Basic Campaigns- and changes to classic combat and capture.
To help all residents of Hisui, check out our Petitions guide, including collecting Night Light Volunteers to get to Spiritomb.
Pokémon Legends: Arceus – Ocean Legend Reward
Once you’ve captured Manaphy, return to Professor Lavender at Base Camps or Galaxy Headquarters.

You will be fascinated to discover that the mystery has been solved and you will be rewarded with three comet fragments. It will be dropped into your item storage if there is not enough space in your Zurrón.

You can sell each Comet Fraction for 15,000P to vendors.
Here are other specific guides for Pokémon: Petitions Arceus Legends: