After you deliver the letter to Steven, Mr. Briney says that the Devon Parts must be delivered to Slateport via Route 109.
Explore the Route
In this route, there are many trainers, but you can avoid all of them and go north to the next area if you prefer.
You can catch Magikarp and Tentacool with the Old Rod here.
The trainers in this area are Sailor Huey, who has Machop; Sailor Edmond, who has Wingull and Machop; Tuber Ricky, who has Zigzagoon; and Tuber Lola, who has Azurill.
In addition, you can go into the Seashore House, where if you defeat all the trainers, you win six Soda Pops, and afterward you can buy Soda Pop there. The trainers in Seashore House are Tuber Simon, who has Azurill; Delinquent Destinee, who has Sableye; Street Thug Blair, who has Poochyena and Carvanha; Beauty Johanna, who has Goldeen; and Sailor Dwayne, who has Tentacool and Machop.
In the northwest corner of the beach, there is a PP Up.
Go north to reach Slateport City.