Fish-men were created by a group of scientists on the Georgia coast after skydark. They were 6-7 feet tall, with scaled skin, webbed and clawed hands and feet, large goggling eyes, an under slung jaw filled with needle teeth, and a crest over the head. They were intelligent and spoke their own language; some could also speak English, after a fashion.
A small colony of them lived in the ocean off the coast, trading with a commune near Kings Point Naval Station. (Watersleep). The companions also ran into a group of Sharkmen in the Florida Keys (Deep Empire); it's entirely possible that the two groups were related. Another group lives in an abandoned nuclear power plant in the Great Lakes; these ones are shorter, but similar in appearance. (Downrigger Drift). They are likely the product of a similar (or the same) experiment in a different location (freshwater vs. saltwater).