After exploring Route 127 and the large underwater trench using Dive, you go south to Route 128, where Seafloor Cavern can be found.
Explore the Route
You can find three hidden Heart Scales in this route, one on each of the shallow water areas. Use your Dowsing Machine to find them.
There are four trainers that you can battle above the water in this route: Ace Trainer Cornelius, Fisherman Wayne, Tuber Delmar, and Tuber Marlene.
There are three separate underwater areas that you can explore in this route.
If you dive in the center of the circle of shallow water, you will reach an underwater area where you can find a hidden Adamant Orb.
If you dive in the deep water surrounding the circle of shallow water, you reach an underwater area where you can find a hidden Draco Plate, Pixie Plate, and Stone Plate.
If you go east from there and dive in the small area of dark water, you find an underwater area where you can battle Free Diver Mayu, and you can find a hidden Protein and a hidden Pearl.
When you are ready to move on, dive into the dark water surrounding the circle of shallow water, then go to the south area to find the entrance to Seafloor Cavern.