There are 7 Diglett to find in the Stepping-Stone Sea.
Alolan Diglett #1, #2
These two Digletts are hiding in the same spot! On the map this small island is opposite the top entrance to Loop Lagoon. Alternatively, if you go to the Tower of Waters in Challenge Beach, face the door, turn left and then cycle on the water this will be one of the first islands you land on. Just keep a look out for the rock.
Alolan Diglett #3
This island is the most northern island of the Stepping-Stone Sea. If you stand in front of the Tower of Waters in Challenge Beach, face the doors and then look out towards the ocean to your left it is the island with two trees on it.
When you land on the island turn left and follow the narrow grassy path. At the end of the path you’ll find the Diglett hiding in front of some bushes/flowers. You’ll also find TR89 Hurricane if you haven’t picked it up already.
Alolan Diglett #4, #5
These Digletts are on the same island as Diglett #3. Again, this island is the most northern island of the Stepping-Stone Sea. If you stand in front of the Tower of Waters in Challenge Beach, face the doors and then look out towards the ocean to your left it is the island with two trees on it.
When you land on the island turn right. The Digletts are hiding near the tree. One is by the edge near the water and the other is by some flowers closer to the Berry Tree.
Alolan Diglett #6
This Diglett is on the same island as Diglett #3,#4 and #5. Again, this island is the most northern island of the Stepping-Stone Sea. If you stand in front of the Tower of Waters in Challenge Beach, face the doors and then look out towards the ocean to your left it is the island with two trees on it.
The Diglett can be found hiding by the tree and Pokemon Den at the back of the island.
Alolan Diglett #7
Diglett #7 is on an island somewhat shaped like an 8 (not apparent on the map). There is a Pokemon Den on one side and a rock on the other. The island is the closest to Workout Sea and in front of the two ocean entry points to Loop Lagoon. The Diglett is hiding in the sand in the middle of the island.