Trainers who are in the market for a well-known Water-type Pokémon can always try to get Feebas. This powerful Pokémon is difficult to catch but worth it. Feebas can evolve into Milotic, but if you want them, it will take some time.
You'll need to have six badges and a good / super rod before embarking on this quest for Feebas, so be sure to work through the rest of the game.
Feebas will only appear in one place in Pokémon Brilliant Diamond and Brilliant Pearl. Head to Mt. Coronet west of Celestic Town on Route 211. You will need the Strength TM as there will be some boulders to move. You can find the HM Force at the culmination of the Lost Tower on Route 209. You must go there and get the HM, and then you will only be able to use it after beating Byron at the Canalave City Gym.
When you can use Force, use it to clear boulders and go down stairs in the mountain. Next, you'll need to use Defog and Surf to get to the lake. Defog can be found at the Great Marsh Gate near Pastoria City by talking to the receptionist on the left. You will have to pay a nominal fee for this HM. Surf can be found in Celestic City after defeating Team Galaxy in the Ruins. Look at the paintings on the walls and Cynthia's grandmother will come in and give you Surf.
As you can see, there is a lot of effort required to get to this point as well. After that, it just gets more annoying. You have to fish to find Feebas and it will only appear on four randomly generated tiles that rotate each day. The good news is that those tiles give you a 50% chance to catch him, which is already something. You will only have to fish in each lake tile a couple of times to try and catch it, unfortunately.