PRICES GO UP AT THE GATE
Combating the dearth of such things in the anime and manga industry, what we have here is a list of anime and manga that feature mermen.
Feel free to add any more that you know of.
This Boy Caught a Merman
Shima, a lonely boy, nearly drowns and is rescued by a handsome merman, Isaki.
Orenchi no Furo Jijo (The Circumstances in My Home's Bathtub) by Itokichi
(mostly) 4-Panel Gag Manga, 4-min Anime Shorts
One day, Tatsumi, who lives alone, rescued a "devastatingly handsome" merman by the name of Wakasa, and allowed him to live in his bathtub. Things spiral out of control from there. The manga is not BL, but hilariously suggestive. The anime on the other hand...
+Anima by Natsumi Mukai
+Anima takes place in a world where children in need are helped by animal spirits who temporarily give them their powers. Husky has a fish +Anima and thus can shift into a merboy form at will. He's "rescued" from his crossdressing job as the "Mermaid Princess" at a circus by Cooro, who has a crow +Anima.
Free! Eternal Summer
Haru is depicted as a merman during the ending credits song, "Future Fish". (Though perhaps he was one all along...)
Possibly not dead.