This is a small area of sea sandwiched between Stepping-Stone Sea and Honeycalm Sea. Honestly, the hardest part of Insular Sea is probably figuring out where the zone begins and where it ends.
1/6: Start from the Tower of Waters in Challenge Beach. Go to the back of the tower and face away from the tower. Swim straight towards the island with a single tree. Upon arrival, check the red flowers to the right of the berry tree.
2/6: From Diglett 1, go clock-wise around the island. When you reach the opposite shore (furthest from the mainland), look for some small pebbles between the shore and the tall grass.
3/6: From Diglett 2, continue clock-wise around the island. Go past the tree, towards the next bed of red flowers. It's burrowed in the sand to the left of these flowers.
4/6, 5/6 and 6/6: From Diglett 3, face the Tower of Waters, then turn slightly left. There should be a Pokémon Den in the ocean. Head for the islet that's past the den and slightly to the left. You'll find not one but two Diglett here! Actually, there's a third Diglett--or rather, a... we'll let you find out.
We're still at 130+ Diglett, but maybe it's time to start counting down instead? You've got 24 Diglett left. That's only the number of hours in a day. Surely it won't take a hour to find each remaining Diglett?