Hellboy: The Third Wish #1 & #2
A darker story with mermaids getting wishes gone horribly wrong.

Robert E. Howard's Savage Sword #3 - Chapter "The Spear and the Siren"
The protagonist finds a mermaid captured aboard a ship.

The Brave and the Bold (1955) #21 - Chapter "The Viking and the Firebird"
A mermaid helps a viking from drowning.

For more story-building that, among a lot of other lore, explains the existence of merpeople in the DC Aquaman universe, I'd recommend the Atlantis Chronicles #1 through #7.

Golden Age Wonder Woman vaguely more "Greek" than she is currently, and one of her love interests as a teenager was a merboy, who eventually was called Ronno.

Superman Confidential Issues #6 - #7 is a hypothetical scenario where the city of Metropolis is sunk underwater. Everyone except Superman is turned into merpeople.

If you like Disney's The Little Mermaid, there are comics (and books) that expand on the world before the first movie.