Your best bet is probably Route 128 in the Hoenn region. When you surf, there is 50% chance you will get a Tentacool. 15% chance you will get a Tentacruel and a 35% chance you will get a Pelipper. Otherwise, if you try to chain fish, you have a 35% chance of getting a Tentacool in that area and is for pretty much every other place. Hope this helps!