The Good Rod can be obtained by speaking to a Fisherman NPC standing on the eastern shore of Route 118.
But since this area is separated by an ocean, Route 118’s eastern side can only be accessed with HM03 Surf. You’ll get that HM from Wally’s father after you defeat Gym Leader Norman in Petalburg City.
Getting Surf for Route 118
After earning the Balance Badge from defeating Gym Leader Norman, you will meet Wally and his father outside the Petalburg Gym.
For helping Wally get his first Pokémon, his father will reward you with HM03 Surf.
Afterwards, Wally will take you with him back to Mauville City.
Good Rod Location (Step-by-Step)
With Surf, it’s now possible to cross to the eastern side of Route 118 where the Fisherman is located.
Step 1: Starting from Mauville City’s east entrance leading onto Route 118, head directly to the east to leave the city.
Step 2: Keep going east until you reach the shore. Then use HM03 Surf to cross the water.
Step 3: Continue heading east while surfing until you reach the eastern side of Route 118 – you should see a Fisherman NPC standing nearby with a fishing rod in hand.
Step 4: Talk to this fisherman to obtain the Good Rod.
Pokémon Obtainable with the Good Rod
Compared to the Old Rod, you can now catch higher-leveled fish Pokémon, and you’ll find much more variety using the Good Rod as well.
Using the Good Rod in various locations listed below, you can catch the following Pokémon at this stage of the game: