Hi. Uhm, I've been through mt. Coronet looking for a feebas for like almost..

The Waterfall is located somewhere else in Mt. Coronet you have to go through the way of Celestic Town. Once you go through that way you are able to go to the waterfall. I am glad to help you.

~Twilight_Sasuke~

1. Feebas is only found in one room in Mt. Coronet. He's found in the room with that big lake to the east; this room is found along the path to reach Route 216, starting from the Mt. Coronet entrance near Celestic Town (or Eterna City). He's only found in 4 random spots (in the water), however, and the spots do change daily.

2. The waterfall in Mt. Coronet is found along the path to reach the top of Mt. Coronet (Spear Pillar). Start at Oreburgh City, go in Mt. Coronet, surf across the water and go up the wall. Continue north in this room, and go in the next room. Follow that path and exit the cave to be on the mountainside. There should be a cave near here; it's a "dead end". Go in this cave, and you should see a waterfall. Go up the waterfall and use the Super Rod in this body of water. Dratini is pretty rare, but Dragonair are even more rare. (If you go in that room to your north, and examine the rock, you'll obtain the Steel Plate, I believe.)

Try fishing from land into water it's easier to appear I caught three but now I have 16 hehe... I could trade it to you but I don't know what my friend code is.
I looked for a whole day then found it on the next.
Hope I helped
JD