Kakatte koi

Castle swimmers though it's not a yaoi lololol it's also free on webtoon

MERMAN
http://www.mangago.zone/read-manga/bakemono_bl/
http://www.mangago.zone/read-manga/wave_nail/
http://www.mangago.zone/read-manga/wadatsumi_no_yometori/
http://www.mangago.zone/read-manga/free_and_the_scales_fade_to_foam_doujinshi/
http://www.mangago.zone/read-manga/kuchibiru_mellow/
http://www.mangago.zone/read-manga/free_dj_tadashii_ningyo_no_aishikata/
http://www.mangago.zone/read-manga/mermaid_s_egg/
http://www.mangago.zone/read-manga/gone_with_the_bubbles/?U77F
http://www.mangago.zone/read-manga/utakata_no_uroko/
http://www.mangago.zone/read-manga/seashell_boy_pearl_cum_boy/
http://www.mangago.zone/read-manga/dolphin_fairy/
http://www.mangago.zone/read-manga/koiseyo_sennen_neet/

Got an amazing chapter right here (⌒▽⌒)
http://www.mangago.zone/read-manga/tell_me_your_wish/mpn/ibx_chapter-17/pg-1/
Also, this is highly recommend http://www.mangago.zone/read-manga/wadatsumi_no_yometori/

And this is the most beautiful merman BL anime ever!!!
https://www.anime-planet.com/anime/this-boy-caught-a-merman