Yes, Feebas can be found on any part of the water on Route 119. However, there are only like 6 "tiles" that Feebas can appear on, and there are probably around 300-400 different "tiles" of water on Route 119. It doesn't matter which rod that you use, it will not affect your chances of catching a Feebas. On the six tiles that contain Feebas, there will be a very high chance of encountering Feebas, but other Pokémon will still appear in "Feebas tiles." Therefore, it is recommended that you encounter at least around 3 wild Pokémon on each tile before you move on and fish at a different one. Feebas hunting can take a really long time and requires a lot of patience. It is best to start at one end or the other of the water on Route 119, and just keep fishing at each and every tile until you get your Feebas.