Dewford is a small seaside town on a small island accessible only through two water routes. Before getting Surf you can only go here by asking Mr. Briney to take you here on his boat. Just north of Dewford is Granite Cave. This town doesn't have a Poké Mart, so you'll want to stock up on items before you come here.
In the house north of the ship dock, talk to the person; he will give you a Silk Scarf. The person to the east of the gym will give you an Old Rod. The Northernmost building is the Dewford Hall, where residents discuss the latest trends in Hoenn. A man inside will give you TM36 Sludge Bomb once you've defeated your fifth gym.
Dewford Gym
Gym Leader: Brawly
Badge: Knuckle Badge
When you enter this gym, it will be pitch black. As you beat more of the trainers in this gym, the illuminated area around you will expand.
This Gym's speciality is Fighting Pokémon. It's weak to Psychic and Flying moves, though some trainers have a Meditite to negate this weakness. Any Flying type or Psychic type would do. Avoid Normal, Rock and Dark types. If you have a Combusken, it would have learned the Flying move Peck before evolving; this move will be useful here. Surprisingly, both of Wurmple's final evolutions are effective here, as they are Bug type which resist fighting moves while they both learn Gust, a Flying Type move, at level 13, which is the ideal level to challenge this gym.
Defeat Brawly and he'll award you with the Knuckle Badge, which makes any Pokémon over level 30 obey you and let's you use HM05 Flash outside of battle. You also get TM08 Bulk Up, which is a great move to teach to a physically oriented Pokémon like Fighting types and Flying Types.
After the Gym
Once you're done with the gym, there's not much else to do but head out to Granite Cave to deliver the letter to Steven if you haven't done so. If you did, talk to Mr. Briney to go to your next destination: Slateport City.
#129 Magikarp
After getting the Old Rod, the most common Pokémon you can fish is Magikarp. Now anyone who's familiar with Magikarp knows that it is absolutely useless. It's first move, Splash, does no damage at all, it can't learn TMs or HMs and it only learns Tackle at level 15. Even battling it isn't worth the trouble since it gives so little experience.
If you're patient and manage to train one to level 20 however, it evolves into Gyarados, a powerful Water and Flying type with some useful moves, including the powerful Hydro Pump and Hyper Beam.
#72 Tentacool
The other Pokémon that you can catch with the Old Rod is Tentacool. As a Water and Poison type, it can learn moves of both types. Just by leveling up it can learn moves like Constrict, Acid and eventually Hydro Pump. Tentacool's strength isn't in how powerful it's attacks are though, it's in its high Special Defense, letting it survive against common Water type, Fire type and Dark Type attacks. It evolves into Tentacruel at level 30.
Items to be Found
Silk Scarf - In the house directly north of the dock.
Old Rod - From the fisherman near the gym.
TM36 Sludge Bomb - From a man inside Dewford Hall after you get five badges.
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