Instructions
Go to Route 30 or Route 31. Both are north of Cherrygrove City.
Walk to one of the bodies of water on the route and use ̶0;Surf.̶1;
Move around on the water until you are randomly attacked by a Poliwhirl. Poliwhirl has a 10 percent encounter rate.
Lower the Poliwhirl̵7;s energy and capture it with a Pokeball. The Poliwhirl is weak to Grass and Electric attacks such as "Vine Whip" and "Thunder."
Go to Route 42, which is west of Mahogany Town. Speak to Tully to fight him and obtain his PokeGear phone number. Tully is dressed in fisherman's attire and can be found near the staircase prior to reaching Mt. Mortar.
Wait until you receive a PokeGear phone call from Tully. He will award you with a Water Stone. This call is randomly generated, so be patient.
Access your item inventory and select the Water Stone. Give the Water Stone to your Poliwhirl to evolve it into a Poliwrath.