Where to find Mr Briney in Pokémon Emerald?

After defeating Team Aqua in the cave, go out of it and go to the Devon Corpaoration building. Mr. Stone is waiting for you. He will have task for you and in return, he will give you Pokénav. Go down from Rustboro and through the Petalburg woods. After you go outside go left then down. Near the beach there would be a hut. Inside it is Mr. Briney with his Peeko playing around.