from wiki and I have taken a few liberties with the story

"Humans crash on a distant planet which is earthlike but completely water-covered; their ship is too damaged to take off, nor do they have sufficient supplies to survive for long. The humans create a race of microscopic aquatic humanoids to carry on their legacy before they themselves die. The majority of the story concerns these creatures and their intelligence, curiosity, and evolving technology. In particular, the aquatic humanoids develop a "space ship", or rather "air ship", which enables them to pierce the previously impenetrable surface of the water and travel through what is, to them, hostile space—open air—to other worlds in other bodies of water."