RE:...

Alright there isn't an exact spot where Feebas is located.
Feebas can only be found on 6 tiles out of the 400 that there are, or something
like that.
The tiles are rearranged everytime the "Popular Phrase" in Dewford is changed.
I can't remember the route number so, I say try your luck on the water next to the house that has a woman with many Wingulls. Check everytile like 5 times.
Use this pattern to cover the whole body of water:
>->->-v
v-<-<-<
>->->-v

Feebas as a POKéMON is fairly weak, however unlike Magikarp is can learn TMs.