i havent read any yaoi mangas where the uke or seme is a mermaid <merman>...... if a...

http://www.mangago.zone/read-manga/junketsu_ningyo/
http://www.mangago.zone/read-manga/wagamama_na_ningyo_sama/
http://www.mangago.zone/read-manga/koi_uta/

@Haze
Check this ff. links:

http://www.mangago.zone/r/l_search/?name=mermaid+yaoi
http://www.mangago.zone/r/l_search/?name=merman+yaoi

Ihope I help you and you will like it :)

Here are a few:
http://www.mangago.zone/read-manga/junketsu_ningyo/
http://www.mangago.zone/read-manga/mister_mermaid/
http://www.mangago.zone/read-manga/wild_fish/
http://www.mangago.zone/read-manga/wagamama_na_ningyo_sama/

hum its not quite a merman but try sex pistols. At some point, there is a story with a boy that is what we can call a fish... or something like that.

Yeah, Sex Pistol. It a good manga too. Love it.

there's this one http://www.mangago.zone/read-manga/wild_fish/ it's funny and romantic hope you like it ^.^