UwU

https://www.mangago.me/read-manga/wadatsumi_no_yometori/
https://www.mangago.me/read-manga/itoshiki_ningyo/
https://www.mangago.me/read-manga/the_worst_mermaid_auction/
https://www.mangago.me/read-manga/the_falling_merman/
https://www.mangago.me/read-manga/mermaid_s_egg/
https://www.mangago.me/read-manga/the_first_love_of_the_sushi_restaurant_owner_is_a_mermaid/
https://www.mangago.me/read-manga/desharow_mermaid/
https://www.mangago.me/read-manga/mister_mermaid/
https://www.mangago.me/read-manga/wave_nail/
https://www.mangago.me/read-manga/see_you_later_mermaid/
https://www.mangago.me/read-manga/umi_no_soko/
https://www.mangago.me/read-manga/warawanai_ningyo/
https://www.mangago.me/read-manga/samezoku_e_no_sasagemono/
https://www.mangago.me/read-manga/gorae_byul_the_gyeongseong_mermaid/
https://www.mangago.me/read-manga/castle_swimmers/ (webtoon)
https://www.mangago.me/read-manga/the_mermaid_i_loved/
https://www.mangago.me/read-manga/augustus_in_the_human_world/
https://www.mangago.me/read-manga/castle_swimmers/
https://www.mangago.me/read-manga/anaerobic_love_prisoner_of_the_deep_sea/

I couldn't find any Prince x mermaid rec, but here's some manga that talk about mermaid

GYATTTT THESE RECS LOOK GOOD

here, pirate x mermaid, straight smut
https://www.mangago.me/read-manga/once_upon_a_time/