As the name suggests, the stage takes place underwater. There are six sections, and being underwater will cause all of Kirby's movements to be slow.
Section 3 in the middle up against the roof, though there is a water current, so if you miss it you won't be able to swim towards it again. Use Rock Cutter combo to turn into Pitch to fly towards the middle of the roof to see the first Shard.
Crystal Shard 1
In section 4, after traversing some of the current you will see a pole. Travel down it to see the next Shard. You will also need the Rock-Cutter combo so the current doesn’t take you away.
Crystal Shard 2
The Blowfish is the Section 5 boss
The blowfish is hard to avoid if you are stuck following the current in the Blowfish's opposite direction while trying to inhale enemies to shoot back at him.
In section 6, a current will, very quickly, pull you to the end of the stage. Use the Rock-Cutter combo to slow yourself down. Now head to the last bottom ditch. You will see the Shard. If you swim past the Shard, you must reset the level.