you have to beat the mossdeep city gym and fight team magma in the space center then you get dive from steven teach it to a Pokemon GO to route126 surf until you find white rocks then you find a dive area and the a little cavern after a little exploring go into the cavern go up and voila
under sootopolis
He is at Sootopolis city...you can battle him only after the sootopolis incident
in sootopolis city
north of sootopolis city
Sootopolis City
Wallace
simpliy the dark spot leading to sootopolis you have to use dive
Go to Sootopolis.
sootopolis city
you go to sootopolis city
underwater near sootopolis city
well i forgot