After seeing a disturbance in Route 128, you head south from Mossdeep City to Route 127, where you can use Dive to explore underwater.
Explore the Route
In the northwest corner of this route, you can battle Bird Keeper Byron.
If you go southeast from the entrance, you find some shallow water where you can battle Fisherman Jonah, Fisherman Henry, and Fisherman Roger. There are some Secret Spots in this area, and you can get a Zinc in the northeast corner.
In the water in the center of this route, you can battle Ace Trainer Hisato, Ace Trainer Claudia, and Triathlete Jaylon.
In the southeast part of this route, there is another area of shallow water. There are more Secret Spots to be found here. You can also battle Black Belt Koji in this area.
In the southwest part of this beach, check on the sparkle to find a Heracronite.
North of the Black Belt, there is an area of dark water that you can Dive into. There is a hidden Heart Scale in this underwater area, but you can't use your Dowsing Machine underwater, so you will have to press A to check each area for hidden items underwater. The Heart Scale is in the middle of the area. You can battle Free Diver Aileen in this underwater area.
Go to the northeast of the area and press A to go back to the surface. This leads you to a new area that is only accessible from underwater. In the middle of the area, there is a Carbos, and in the north, there is a Rare Candy.
Go back underwater to reach the middle of the route again. You can dive in the dark areas of the main route to explore the underwater trench there. There are six hidden items to be found in the main trench: a Dread Plate, an HP Up, an Insect Plate, a Red Shard, a Spooky Plate, and a Star Piece. You can also battle two trainers in the main trench: Scuba Diver Javier and Free Diver Kailyn.
When you are ready to move on, go south from Route 127 to reach Route 128.