Instructions
Acquire the Super Rod. In "Pokemon: Sapphire," the Super Rod can only be used to enter into random encounters against Feebas and Carvanha, removing the possibility of encountering other types of Pokemon. Speak to the fisherman in Mossdeep City to get the Super Rod.
Travel to Route 119, located west of Fortree City and north of Route 118 in the Hoenn region. You should have a Pokemon that knows the ability "Surf" in your party. Surf over the water while you have the Super Rod equipped. Explore one column of water at a time.
Enter into several random encounters along one column of the water. If none of these are encounters with Feebas, proceed to the next column and repeat the process. If you do encounter Feebas but did not defeat it, continue exploring the same column of water until you find Feebas again.
Travel to Dewford Town if you grow frustrated with finding Feebas. In the water along Route 119, Feebas will only appear randomly on six tiles at a time, out of hundreds on which you can put your character on. Travel to Dewford Hall and examine the sign in front of the building to change the "trendy phrase." This will randomly shuffle the six tiles on which Feebas appears.
Return to the water along Route 119 and continue to explore the water, one column at a time. You may frequently return to Dewford Town to change the trendy phrase to improve your odds of discovering Feebas.