Grab the Poke Ball to the north then head east. You'll encounter a Team Aqua grunt with Mr. Briney's kidnapped Peeko. Talk to him again to challenge the grunt to a battle.
Team Aqua Grunt (Prize Money: P520)
After defeating the grunt, you'll acquire the stolen Devon Parts. Peeko will also be reunited with Mr. Briney. He'll now return home to his cottage in Route 104. You won't be able to proceed any further for now, thanks to the boulders blocking your path. For now, backtrack to Rustboro City.