There are 6 Diglett to find in the Insular Sea.
Alolan Diglett #1
Despite what the map implies, this island is directly behind the Tower of Waters. The Diglett is hiding by some pebbles and flowers. Use the tree and Pokemon Den as reference points.
Alolan Diglett #2
This Diglett is on the same island as Diglett #1. Again, despite what the map implies, this island is directly behind the Tower of Waters.
The Diglett can be found hiding on the shore.
Alolan Diglett #3
This Diglett is on the same island as Diglett #1 and #2. Again, despite what the map implies, this island is directly behind the Tower of Waters.
The Diglett is hiding in a flower patch near the Berry Tree.
Alolan Diglett #4, #5, #6
These three Digletts are hiding on a small sandy island with only a Pokemon Den on it. It is close to the island explained above. Its position is between that island and Honeycalm Island. While it can be seen from the shore by Tower of Waters, the distinct Pokemon Den is not visible from that distance.