Not much has been revealed about there creation, because there is nothing in any Marvel comics database that says they were created by the Celestials, so if not created by them, maybe by magic, like the Darkhold for instance. Or perhaps by Gaia at sometime before the Great Cataclysm.

Mermaids and humans maybe.

I always thought they were the descendents of the original Atlanteans

I'm gonna have to go with magic, because they obviously weren't created by the Celestials. If I had to guess, i'd say they were created as a last resort to save the Atlantean race, which would explain why they are descendants of the original Atlanteans.

I think they are an evolution of the human race.

If I had to guess, i'd say they are water breathing Eternals, like the Lemurians are water breathing Deviants.