You must have a pokemon with move DIVE to get into Sootopolis City... First go to Route 126 by using SURF, then DIVE into the deep parts of the sea, now go south underwater 'til you see a cave, that cave is the entrance to Sootopolis City...
in sootopolis city
sootopolis city
sootopolis city
underwater near sootopolis city
Sootopolis city.
Underwater around sootopolis city and mossdeep city
Underwater, in the grass outside Sootopolis City.
sootopolis city
Sootopolis city but you cannot go in.
you dive at route 128 by sootopolis city
It's in the Cave of Origin in Sootopolis city.
In the sootopolis city gym.