After defeating your father Norman in the Normal-type Petalburg Gym, you can Surf east from Mauville City to Route 118.
In the tall grass of Route 118, you can catch Zigzagoon, Linoone, Wingull, Electrike, Manectric, and (rarely) Kecleon. While Surfing in Route 118, you may encounter Tentacool, Wingull, or Pelipper. If you use the Old Rod here, you can encounter Tentacool and Magikarp. If you use the Good Rod (which you get in this route from the fisherman on the east side), you can find Tentacool, Magikarp, and Carvanha. If come back here after getting the Super Rod, you can find Carvanha and Sharpedo.
On the west side of Route 118, you can encounter Aroma Lady Rose, who has Shroomish and Roselia (level 16 in Ruby and Sapphire, level 14 in Emerald); Fisherman Wade, who has a level 17 Carvanha in Ruby and Sapphire, and a level 16 Tentacool in Emerald; and Guitarist Dalton, who has level 15 Magnemite and level 15 Whismur. In addition, in Emerald, you can encounter Youngster Deandre, who has a level 14 Zigzagoon, level 14 Aron, and level 14 Electrike. He might join Rose in a double battle.
On the east side of Route 118, you can encounter Interviewer Gabby & Ty, who have a level 27 Magnemite and level 27 Loudred; Fisherman Barny, who has two level 27 Carvanhas in Ruby and Sapphire, and level 25 Tentacool and Carvanha in Emerald; Bird Keeper Chester, who has Taillow and Swellow (level 26 in Ruby and Sapphire, level 25 in Emerald); and Bird Keeper Perry, who has Wingull (level 27 in Ruby and Sapphire, level 26 in Emerald).
Search the rocks on the west side of the Route to find a Heart Scale. Search the rocks on the east side of the Route to find an Iron. Talk to the Fisherman on the east side to get the Good Rod.
When you go farther east, Steven will appear. He will talk you for a little while, then leave.
There is a tree that you can cut down if you want a shortcut. There are Sitrus Berry trees north of that tree. Go farther east and you can find a Hyper Potion on the far east side of the route.
Go north to reach Route 119.