Mermaid's egg is pretty good
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/
bunch of merman yaois onegai desu