Pokemon Legends: Arceus brings together hundreds of Pokemon in a new open setting that lets you capture most monsters at will, but some especially rare ones are gated behind story progress or special quests. This is the case for the Manaphy, a mythical water-based Pokemon that debuted in Diamond and Pearl. You can get a Manaphy by completing the side quest The Sea's Legend, but this quest is anything but straightforward. Here's how to complete The Sea's Legend and claim your Manaphy.
In Pokemon Brilliant Diamond and Shining Pearl, you can find a book in the Canaleve that also describes this legend. That book references a party consisting of a Mantyke, Buizel, and a "Qwilfish with huge spikes." So to start, you'll need to capture those three Pokemon: a Mantyke, a Buizel, and a Qwilfish.
Unlike some Pokemon with leveling requirements, Qwilfish only evolves if you use a specific move in a specific way. Its Pokedex entry hints at the conditions, having one criteria for both using Barb Barrage and another for using strong style moves. So you'll need to use Barb Barrage in strong style 20 times. Your Qwilfish will also need to be at least level 20, but most that you capture in the wild will fulfill that requirement anyway. Once you've completed the criteria, evolve your Qwilfish.
Before you head over, use the camp to sleep until evening time. Then head for the pincer-like rocks and use your Pokemon ride to sail right between them. If you've fulfilled all the conditions to this point, you'll see a prompt that you heard a boulder shifting in the distance.
Head back to camp to swap out your party for stronger Pokemon if you need to for a tough battle, and then head to the Seaside Hollow cave, just northwest of Veilstone Cape. That's where you'll find Manaphy.
You can capture any or all of the Pokemon here, but only Manaphy is needed to complete the side quest. If you defeat it by accident, leave the cave and come back in to re-initiate. Once you've captured it, you'll get a prompt saying that phase of the quest is complete.
Report back to Professor Laventon and he'll give you three Comet Shards. Plus you'll be one step closer to catching every Pokemon, a necessary step to complete the lengthy post-game quest.
For more about Pokemon Legends: Arceus, check out our how long it takes to beat and how missions work, how to release multiple Pokemon, and how to catch Shiny Pokemon. Plus, make sure to read our Pokemon Legends: Arceus review.