Sea Serpent, they merely discuss mermaids:
"Ah, you are talking about mermaids then, the cold fishy women with clammy kisses and scaly breasts? Ladies who lure you by clasping you to their cold bosoms and then make you go 'Now I am dead oh dear oh me' by dragging you beneath the briny?"
-Neil Gaiman, The Sandman: Worlds' End (New York, 1994), p. 76.
Ironically, The Sandman is set in a fictional universe that does include merpeople as descendants of Atlanteans although they do not appear here. This dialogue is ambiguous: maybe mermaids are real; maybe they are not. Cold, fishy, clammy, scaly murderesses do not correspond to the merwomen in Poul Anderson's The Merman's Children but do exactly match Dahut in Poul and Karen Anderson's The King Of Ys.
Poul Anderson, of course, presents an sf version of merpeople (see here) and also alien aquatic beings who, like the merpeople, dance in the water in Ensign Flandry - also human beings who remain air breathers but spend most of their lives on and under water in "The Game of Glory." Andersonian comprehensiveness.
The Ysan sea god is Lir. The mermen's city is Liri. Superman's mermaid girlfriend is Lori Lemaris, which sounds similar.