Just two more zones remain before you can return to your day job. There are only a measly 3 Diglett here, but it's not finding them that's hard--it's trying to find a place where you can land.
1/3: Start from the Tower of Waters in Challenge Beach. With your back to the entrance, turn left and swim into the water. Go straight ahead, while keeping close to the mainland on the right. When you're parallel to Honeycalm Island, there's a secluded shore. It's burrowed to the right of the Pokémon Den here.
2/3: From Diglett 1, go further into the enclosed area. It's next to some pebbles just past the gold item ball.
3/3: From Diglett 1 and 2, head into the sea and swim until you're halfway to Honeycalm Island, then stop. Now turn right and travel while parallel to the mainland. You should reach an islet between a trio of rocky islands and the mainland. There's nowhere it can hide...
Only 11 Diglett remain. We're almost at single digits now!