1. Defeat the Elite Four and become the Champion: This unlocks the ability to fish in all locations.
2. Talk to the Old Fisherman in Slateport City: He'll be standing near the entrance to the port.
3. Complete the "Finding the Old Rod" sidequest: This involves helping him recover his lost fishing rod.
* Follow the Old Fisherman to the fishing spot near the lighthouse.
* Find the Old Rod hidden in the grass.
* Return the rod to the Old Fisherman
4. He will reward you with the Super Rod.
The Super Rod allows you to catch rare and powerful Pokemon that are not available with other fishing rods.