Not every Pokémon rushes out at you in the grass in Pokémon Sword and Shield. Some of them take time to locate, and you need to go out of your way to add them to your collection. For example, if you want to add Feebas and its evolution Milotic to your Pokédex, you need to spend a great deal of time fishing.
You can find Feebas in Pokémon Sword and Shield, but you don’t have the best chances. It shows up on Route 2, and you need to toss your fishing rod into the water along the route to reel it in and battle it. Unfortunately, you only have a one percent chance to capture it on this route. There are ideal fishing spots on this route to the west of the Professor’s house, and to reach them, you need to use the Rotom Bicycle upgrade to ride across the water.
This percentage chance means you need to spend a great deal of time at a watering hole along this route to find it. Because it has a low probability, it spawns in between levels 37 and 40, so it’s a decent battle. Feebas is a pure water type, so you may want to avoid using Electric and Grass-types to fight it, to ensure you don’t cause it to faint. After you capture it, evolving it into a Milotic takes some work.
To add Milotic to your Pokédex. You have the opportunity to find a Prism Scale and have the Feebas hold onto it. While it’s holding the item, trade it with a friend of yours, and it should evolve into the Milotic. Hopefully, your friend has the good faith to offer it back to you. You should have it in short order and added to your collection and your roster.
You cannot locate Milotic in the water. You do need to trade it with the Prism Scale.
Given how long this process can take, locating a Prism Stone and trading it to a friend seems like the best solution to add Milotic to your Pokédex.