After the Gym, a scientist will be robbed by a Team Aqua/Magma Grunt. Chase them north to Route 116. You'll find the Grunt in Rusturf Tunnel, holding Mr. Briney's Peeko hostage. Defeat the Grunt to retrieve the Devon Goods and the stolen Peeko. Mr. Briney will then offer to sail you to Dewford Town, but first, return to the Devon Corp. building in Rustboro.
