This is it: the final zone! To reach here, from the final Diglett in Honeycalm Sea, turn around until you see an island with a singular tree. Then swim in that direction. Alternatively, you can do the same thing from the Towers of Water; Honeycalm Island will be located towards the left. The island is comprised of a middle part and six "petals", like a flower.
1/11: Start from the "petal" with a boulder that's nearest the mainland. Look for a Diglett a few paces away from the boulder, in the direction of the very faraway Tower of Darkness.
2/11: From Diglett 1, stay on the same "petal". Where Diglett 1 was, face the Tower of Waters. The second Diglett is a short distance to the left of the Pokémon Den that's here.
3/11: From Diglett 2, go clock-wise around the island to the second "petal". Go up to the Pokémon Den and face the Tower of Waters. It's on the right side of the yellow flowers just above the den.
4/11: From Diglett 3, head for the third "petal". There should be a boulder here, but the boulder's a red herring. Near the middle of the island, check where the red and yellow flowers meet.
5/11: From Diglett 4, continue to the fourth "petal". This one's next to some shrubbery past the Pokémon Den, away from the island's centre.
6/11: From Diglett 5, move onto the fifth "petal". The first one here is located in the middle of the sand, close to the island's centre.
7/11: Remain on the "petal" where Diglett 6 is. Head past the Pokémon Den, towards the ocean. It's on the shore, in the direction of the sea.
8/11: Go to the final "petal". This one's also on the shore, along the left side if your back is to the island's centre. Only a few more to go! Wahoo!
9/11: Head for the centre of the island, where the big tree is. There's one among the red flowers in the direction of Diglett 8. Pluck it out and feel proud.
10/11: Plus another a few paces away from Diglett 9, if you walk clock-wise. It's between two bunches of red flowers and some yellow flowers. Time to start planning what to do with your soon-to-be freedom.
11/11: ...And another if you continue walking clock-wise for a few more paces. It's between some yellow flowers and red flowers near the tall grass. Go over and dig up that last blight on humanity.
Congratulations, you've found all 150 Diglett! 151 if you count the first one. Your reward is, wait for it, an Alolan Diglett. If you'll excuse us, we're going to bury this reward somewhere we're never going to see it.