Diverse forms of swimmers included eel-people, huge, whale like behemoths, decorative people who swam by squirting water out of their hypertrophied mouths and horrifying multitudes of brainless wallowers that served as food stock were put on an ocean planet. All of them went extinct when the Qu left.
The only species of swimmers that survived were a few forms that were only slightly mutated and resembled their human ancestors to some degree. They had no artificial gills, hand-like flippers, and their hindflippers were still paired like the legs of their ancestors. Even though they didn't understand each other in any sapient way, they spoke to each other.
Over thousands of years, they evolved more streamlined forms and became better and better hunters of fish. It took them a really long time to found a civilization once again, but eventually they managed to domesticate some aquatic organisms and became the Tool Breeders.
- All Tomorrows, by C. M. Koseman (2008)