Poliwhirl has two evolutions in Pokémon Brilliant Diamond and Brilliant Pearl. You can choose to evolve into a Poliwraith or a Politoed. Both are good choices, depending on what you want to use on your trip. However, if you are looking to complete your PokéDex, you will need to find two Poliwhirls and evolve one of each. In this guide, we will share how you can evolve Poliwhirl into Poliwrath in Pokémon Brilliant Diamond and Brilliant Pearl.
You can only evolve a Poliwhirl into a Poliwrath by giving it a Water Stone. You can find a Water Stone by searching for one on Route 213. However, many Pokémon in the game require Water Stones. So you will need more.
The other location where you can find a Water Stone is the Grand Underground. You will be able to access this location by receiving the Explorer Kit in Echoes when you talk to the old man who lives near the Pokémon Center. You must then complete the fossil digging minigame and you have a chance to find one. There is no guarantee that it will appear while you do this. If you want to increase your chances of finding a Water Stone, try increasing your Diglett and Dugtrio bonus by running through the sparks they leave as you move through the passages.