You actually don't really find Wally, your old pal, in Sootopolis. You either have to try finding him in the west part of Sootopolis, or try resurfacing and Surfing more to the west. I dont really know where Wally actually is, but if you find out, contact me at crazykim22@gmail.com.
The only way to reach Sootopolis City is to go underwater using Dive on the route right by the city. When your underwater look for a hole inside a cave and go inside then use Dive to resurface into Sootopolis City.
Wally will be in Petalburg, when you go to battle your father, Norman. Wally will say he's moving and he wants to talk a Pokemon along. He'll catch the Pokemon Ralts, then he'll return to the gym. You can also find him in verandenturf town, in Wanda's house. He runs off and later on in the game, you can find him on Victory Road, located in Ever Grande City.
go to sootopolis city travel west til you find a small town
IN HIS GYM FOR CRYING OUT LOUD!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! Ok, to the idiot who answered the question first: HAVE YOU EVEN PLAYED POKEMON EMERALD? HAVE YOU TRIED TO EVEN GET INTO THE GYM? I BET YOU HAVEN'T BECAUSE HE ISN'T FREAKIN IN HIS GYM. YOU GOTTA FIND THE IDIOT!
you just need to find it and beat team aqua.so you can go to Mossdeep city.
In Pokemon Emerald, you encounter Wally at the end of Victory Road, the last Trainer challenge before the Ever Grande League. After that, he can be challenged daily or so, his Pokemon gaining a few levels, in the same location as before.
At Sootopolis City.
You find it underwater around sootopolis city.
He is the sootopolis city gym leader.
sootopolis city
In the sootopolis city gym.
Underwater near sootopolis city.
in granite cave in sootopolis city
Underwater right beside/infront of sootopolis city
You battle Kyogre in the Cave of Origin, in Sootopolis City.
You find relicanth underwater near sootopolis city.
you have to go to the cave of origon in sootopolis city
in sootopolis city