There are 3 Diglett to find in the Honeycalm Sea.
Alolan Diglett #1
This well hidden path is opposite Honeycalm Island. If you fly to Tower of Waters you’ll be facing Challenge Beach from the tower. Head leftward on the water with your bike and follow the cliff around the island until you reach this path. The Diglett is hiding in the grass/flowers opposite the Pokemon Den.
Alolan Diglett #2
Further up from Diglett #1 is Diglett #2. It is at the end of the path behind TR19 Tri Attack.
Alolan Diglett #3
This Diglett is hiding on the small sandy island that sits between Honeycalm Island and Traning Lowlands’ beach.