Aquatic Men

 

In Poul Anderson's The Merman's Children, merpeople exist because the novel is historical fantasy.

In Anderson's "The Horn of Time the Hunter," human colonists of an extra-solar planet have adapted to become sea dwellers.

In James Blish's The Seedling Stars, one group of Adapted Men lives in pools of water on an extra-solar planet.

In James Blish's and Norman L. Knight's A Torrent Of Faces, the Tritons are genetically engineered human amphibians on Earth.

Maybe that fourth group is the least implausible. They colonize Terrestrial oceans, not other planets.