After witnessing the reawakening of Groudon/Kyogre at the bottom of the Seafloor Cavern, you to go Route 126 on your way to Sootopolis City.
Explore the Route
If you wish, you can explore this route before entering Sootopolis City.
You can battle the following trainers above the water in this route: Ace Trainer Leopold, Swimmer Brenda, Ace Trainer Harriet, Swimmer Dean, and Swimmer Barry.
If you dive underwater, you can battle Scuba Diver Tristan, Scuba Diver Yutaka, and Free Diver Jillian.
There are also many hidden items underwater: a Big Pearl, an Iron, a Mind Plate, a Pearl, a Stardust, and a Yellow Shard.
While underwater, if you go to the far west underwater area and surface there, you will reach an area of Route 127 that you can only access from underwater. In that area, you can battle Swimmer Nikki, and you can go north to find a Secret Spot.
Also, if you are underwater and go to the northernmost area, and go to the surface from there, you will reach an enclosed area in the north of the route. Surf northeast and then dive in the dark water in the northern part of the route to reach a separate underwater area. You can find a hidden Heart Scale and Ultra Ball here.
Go to the west side of this northern trench and surface there to find a Secret Spot.
From the surface of this route, you can go to the southwest area and dive there to reach another separate underwater trench. Go southwest in that trench and go to the surface from there to find an exit from the route that leads to the Secret Islet, which contains a Secret Spot.
When you are ready to continue, Surf to the south side of the central crater and dive there. Then go north into the entrance that you find there, and go up to the surface from there to reach Sootopolis City.