There are a whopping total of 18 Diglett in the Isle of Armor's first zone. Conveniently, exactly half of the Diglett are distributed before the bridge in the centre; the remaining half are found after the bridge. We recommend beginning from Armor Station and searching for the first 9 Diglett, before crossing the bridge.
1/18: Immediately after exiting Armor Station, behind the boxes on the right.
2/18: Next to the tree nearest to Armor Station.
3/18: Next to second tree nearest to Armor Station.
4/18: Turn right after exiting Armor Station and follow the shoreline. In second patch of red flowers.
5/18: Turn left after exiting Armor Station and follow the shoreline. On the sand, near where the beach ends.
6/18: From Diglett 5, go to the right of the nearby rocky hill. By the edge of the cliff, next to two pebbles and before a Pokémon Den.
7/18: From Diglett 6, go to the Pokémon Den by the cliff edge, then follow the river left. Next to some small pebbles, just before the tall grass.
8/18: From Diglett 7, go towards the other rocky hill. Near a smaller rock on the right side of the hill.
9/18: From Diglett 8, face the Master Dojo. Go left towards the river cave entrance. On the outcrop of land, next to the Sun Stone.
10/18: After crossing the bridge, in the middle of a cross-shaped patch of dirt immediately on the right.
11/18: Turn right after crossing the bridge. Found at the end of the outcrop overlooking the sea. There may be a Lopunny near by.
12/18: After crossing the bridge, next to some pebbles along the path to the Master Dojo.
13/18: Just before the exit to the Soothing Wetlands, next to the tree along the left side.
14/18: After going up to the raised land in front of the Master Dojo, behind the rock immediately on the left.
15/18: From Diglett 14, while on the raised land, head in the direction of the wetlands. At the very edge, overlooking the wetlands.
16/18: At the back of the first farming plot, in front of the Master Dojo, next to some pebbles.
17/18: Inside the second farming plot, in front of the Master Dojo. The corner facing away from the dojo.
18/18: From Diglett 17, head in the direction of the ocean. Next to the big tree at the end of the plateau.
With 10+ Diglett rescued, you can obtain an Alolan Meowth and a Kantonian Slowpoke. The Slowpoke even has a King's Rock attached, so you can trade it to evolve it into Slowking!