After you visit your Dad in Petalburg City and help Wally catch a pokémon, you can go west to Route 104.
In Route 104, you can find Wurmple, Taillow, and Wingull. Also, in Ruby and Sapphire you can find Zigzagoon. In Emerald you can find Marill and Poochyena.
The trainers in the southern part of this route are Youngster Billy, who is on the beach (Ruby/Sapphire: Seedot Lv 6, Taillow Lv 8, Emerald: Zigzagoon Lv 5, Seedot Lv 7); and Rich Boy Winston, who has a level 7 Zigzagoon.
Explore the Route
North of Mr. Briney's cottage, you find a tree with Oran Berries and Pecha Berries.
North of Rich Boy Winston, you have to go through Petalburg Woods to continue.