You could have just asked this in the other thread, and it's best not to double post. I'll answer your question anyways...
Surf to the outskirts of Sootopolis City (or Route 126 - whatever you want to call it). From there, have a Pokemon use Dive, and you will dive into the water. (Note: to do this, you must be in a darker area of water) Once you have dived, search around the seaweed (that looks like grass) and eventually you will find one.
Question answered... I think :D
~ MCD