They are creating a vessel to leave their world. Turns out they are building an amphibious ship that can cross a small ridge of sand or dirt from their puddle to the next puddle.
And that they were created by a human explorer sent to terraform a primitive muddy planet. He crashes and all he has are his own germ cells and DNA to use his lab to create a race of very tiny aquatic humanoids.
Surface Tension, James Blish, Galaxy Science Fiction, 1952.
From Wikipedia:
A human colonization ship crash-lands on a distant planet which is Earth-like but whose only landmass is completely covered in shallow puddles of water and mostly microscopic life forms. Normal humans could not survive on this planet, so the crew must genetically engineer their descendants into something that can survive. (Blish coined the term pantropy to refer to this concept.) They create a race of microscopic aquatic humanoids to complete their mission and colonize the planet.
The majority of the story concerns one group of these genetically engineered colonists and their intelligence, curiosity, and evolving technology. In particular, the tiny aquatic humanoids develop a "space ship", or rather "air ship", which enables them to pierce the previously impenetrable surface tension of the water and travel through what is, to them, hostile space—open air—to other worlds in other puddles of water.
You can read it here.
