In the soggy wetlands, there are a grand total of 20 Diglett to search for. We're ashamed to admit how long we spent here... This is probably the hardest place to find all the Diglett. But if you can find all of them here, things should hopefully get easier (but it still won't be easy).
1/20: Next to skinny tree immediately on the left of the entrance (via the Fields of Honor).
2/20: From Fields of Honor entrance, head towards the central pool directly ahead. This Diglett's behind the tall yellow grass that's in front of the pool, near the Pokémon Den.
3/20: With the Fields of Honor behind you, go through the central pool referenced in Diglett 2. This one's next to some shrubbery next to the cliff wall, near a red flowerbed.
4/20: From Diglett 3, turn left while facing the cliff wall. There are a bunch of rocks and fallen logs. Number 4 is behind them.
5/20: From the Fields of Honor entrance, turn right and go past two patches of tall grass, towards the direction of Brawlers' Cave. Suspect 5 is in the middle of some shrubbery before some tall yellow grass.
6/20: From the Fields of Honor entrance, turn left and go past the skinny tree. Keep to the right of the tall grass on the left. As the tall grass ends, there's a shallow pool on the right. Check the yellow flowers above the pool.
7/20: From Diglett 6, look left, towards the edge of the wetlands. There should be a lonesome log by the cliff wall. Search the area in front of it to find lucky 7.
8/20: From Diglett 7, face the log/cliff wall. Turn right and follow the wall. Go past the long stretch of tall grass and the tall vegetation on the left. Before the next shallow pool, search in front of the tall yellow grass.
9/20: From Diglett 8, turn right so the cliff wall is behind you. There should be a skinny tree ahead, with a red flowerbed next to it. Go up to that tree and search above the red flowers/left of the tree.
10/20: From Diglett 9, face the far-off tower. Walk in-between the cliff wall on the left and the nearby shallow pool. As the shallow pool ends, it's hiding next to three small pebbles by the cliff wall.
11/20: From Diglett 10, face the tower in the distance. There's some tall yellow grass on the left and another bunch on the right, further ahead. Go up to the second tall yellow grass and search the pebbles a few paces before it.
12/20: From Diglett 11, turn left (while facing the tower) and travel parallel to the cliff wall. As you get near Courageous Cavern, there's a shallow pool on the left side. Past the pool, look behind the fallen log.
13/20: From Diglett 12, face the direction of the Forest of Focus (the cave should be behind you). Along the left side, there's low-lying land by the river. Walk past this section and search the red flowerbeds near the river.
14/20: Head towards the entrance of the Forest of Focus, keeping close to the riverside. After going past the big patch of tall grass by the river, check the red flowerbed below the tall vegetation.
15/20: From Diglett 14, walk past the tall vegetation, towards the rocky wall. Slowly follow the wall to the forest. Just before the tree, look around the left bunch of yellow flowers.
16/20: From Diglett 15, go towards the tree by the forest entrance. Past the tree, check the right side of the tall yellow grass in front of the forest.
17/20: Stand so the entrance to the Forest of Focus is behind you. Ahead and towards the left, there's a tree bigger than the rest. Go that way. The thing you're looking for is behind this tree, in the middle of the pool.
18/20: From Diglett 17, face Brawlers' Cave. There should be two patches of tall yellow grass just ahead. Go up to the tall yellow grass on the right. The pesky thing is just above the left-most part of this bunch of grass.
19/20: From Diglett 18, face the wall on the left (while looking in the direction of Brawlers' Cave). There's some tall grass with red flowers in front of it. Go up to the grass and search the yellow grass on its left.
20/20: From Diglett 19, while you're still facing the nearby wall, towards the left, there's a red flowerbed that's parallel to a berry tree. Comb the edges of this flowerbed to find the last runaway mole.
That's 30+ Diglett returned to their rightful owner. You can now receive an Alolan Vulpix and Alolan Sandshrew.