After Dewford, Mr. Briney will take you to Slateport City. Explore the city, visit the Oceanic Museum (where you'll have another encounter with Team Aqua/Magma), and then head north through Route 110 to Mauville City. Mauville City is a bustling hub and home to the third Gym Leader, Wattson, who specializes in Electric-type Pokémon. Electric types are strong against Flying and Water, but weak against Ground. Before challenging Wattson, ensure your Pokémon are around Level 20-24. A strong Ground-type Pokémon like Geodude (evolved into Graveler) or a Nincada (evolved into Ninjask/Shedinja) can be very effective. Defeating Wattson earns you the Dynamo Badge, allowing you to use HM06 Rock Smash outside of battle, and TM34 Shock Wave.
