I remember reading a manga about where a guy was extremely disgusted or afraid of fish despite being a fisherman’s son and living near a port. The guy falls into the ocean and meets and is saved by a mermaid or a fish disguised as a human girl and must marry a fish girl so they can be queen or...
This seems to fit
https://mangadex.org/title/37262/daites-ryou-koubouki
Others you may like
https://mangadex.org/title/39432/marielle-clarac-s-engagement
https://mangadex.org/title/43484/i-m-an-opportunistic-princess-in-charge-of-solving-things