The sea-dwelling race in H. P. Lovecraft's “The Shadow over Innsmouth” is the Deep Ones. They are amphibious humanoid beings who live beneath the ocean and worship powerful entities, especially Dagon and Mother Hydra.
In the story, the Deep Ones are connected to the isolated Massachusetts port of Innsmouth. The town's decaying appearance and strange population reflect a pact made by local residents with the creatures: in exchange for prosperity and abundant fish, the people offer worship and intermarry with the Deep Ones.
Lovecraft describes the hybrid descendants as gradually developing distinctive features, including bulging eyes, flattened noses, and a tendency toward a hunched posture. Those who inherit the Deep One nature eventually leave the human world and join the creatures beneath the sea.
The Deep Ones are not the same as the Great Old Ones, a broader category that includes Cthulhu, nor are they the extraterrestrial Mi-Go from “The Whisperer in Darkness.” They are a specific aquatic race central to the Innsmouth story.