Instructions
Walk south from Violet City to reach Route 32. Go inside the Pokemon Center and speak with the fisherman. He will hand you the Old Rod. You can only capture Magikarp, the weakest water Pokemon, with the Old Rod.
Head south on Route 39 to enter Olivine City. Enter the house north of the Pokemon Center. When you talk to the fisherman inside, he will give you the Good Rod. The Good Rod lets you capture better water Pokemon, such as Chinchou and Tentacool.
Walk east from Fuchsia City to reach Route 15, also called the Silence Bridge area. Continue east, and then head north at the end of the road. Turn right when you reach a dead end to enter Route 12. Go into the house on Route 12 to get the Super Rod from the fisherman. With this, you can catch the best water Pokemon, like Qwilfish and Remoraid.