A Team Magma/Aqua Grunt stole the Devon Parts and are blocking the path through Rusturf Tunnel. He even stole Mr. Briney's Peeko!
Stop the Grunt
You can only catch Whismur in this cave.
Go north and go up the stairs to find a Poké Ball.
Go east and you will find the Team Magma/Aqua Grunt who stole Peeko. He has a Poochyena. After you defeat him, he gives you the Devon Parts and leaves the cave. Then Mr. Briney comes and gets Peeko, then returns to his cottage in the south part of Route 104.
Go Back Outside
To the east, there is a dead end that you will only be able to pass after you get a new move for your pokémon. For now, go back the way you came and leave the cave.
Return the Parts
Go back to Rustboro City and go toward the Devon Corporation building. The researcher will thank you and give you a Great Ball. Then you will go into the Devon building, and the researcher asks you to talk to the president of Devon Corporation.
The president, Mr. Stone, upgrades your PokéNav Plus to add the BuzzNav. The BuzzNav activates the StreetPass features of this game, allowing you to send information and your Secret Base to other players with StreetPass. In addition, if you connect to the Internet, the BuzzNav can give you more information.
Then Mr. Stone asks you to deliver a letter to Steven, who can be found in Dewford. Since Dewford is across the sea, Mr. Stone asks you to talk to Mr. Briney in Route 104.
Go to Mr. Briney's Cottage
Go south from Rustboro City and go through Petalburg Woods to reach the south part of Route 104. You can find Mr. Briney at the cottage there. Speak to him. If you're not ready to go to Dewford, you can say Never Mind. Otherwise, choose to head to Dewford Town.