OK...
Go to Dewford, Change Phrase to anything, then go to route 119, keep going until you reach a house and on the right there is water and there are 6 sides facing the water.
Next surf into the water and press left until you can go up, press up only twice.
Now get the super rod, you should easily find Feebas on the 1st thur 10th time.
Good Luck!