Walkthrough

1. For completing this request, you can get Suck Move as a reward.
2. Talk to Piplup in Bleak Beach and follow it near the entrance. They'll ask you to make a waterfall in that area, but you currently don't have the ability to do that yet.
3. Make a Marshy Tall Grass in the muddy waterfall area in Bleak Beach. This spawns Paldean Wooper, who will teach you how to suck up water.
4. Talk to Paldean Wooper to learn how to suck up liquid. With this ability, you can now extend water or create water sources.
5. Demonstrate the new ability by sucking up the muddy water to the pond in front of it. This makes a new source for swamp, and if you put tall grass around it, there is a high chance you can spawn a Clodsire.
6. Go back to Piplup and complete the quest by spitting out water in the area to make a waterfall.

Walkthrough source: Game8.