Trying to locate a Sci Fiction story about a humanoid micro-sized man who swims together with flagellates in liquid media. He moves from pool to another in a submarine he constructs. Probably written by James Blish.
"Surface Tension" by James Blish, perhaps?
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.
It was the first result in Google for
"james blish" submarine