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To my memory, there are 6 possible tiles on that route where Feebas can spawn.
According to Bulbapedia's entry on Route 119:
"There is no known easy method to determine which tiles contain Feebas: the only way is to fish repeatedly in each and every water tile on the route, doing so enough times to be reasonably confident that no Feebas are in that tile (or else risk missing a tile)."
It also notes that the 6 tiles change every time the trendy saying in Dewford Town changes.
So your best bet is to manually change the trendy saying, then fish up 2-3 Pokémon on each tile until you happen to find a Feebas.
So glad they made it so you just need to fish beneath the bridge in ORAS.
Man, I remember Feebas being particularly nightmarish to not only encounter in the wild, but also to evolve, back when I used to get every non-legendary Pokémon in Gen 3.