
I know a few, depending on what genres you like. Here's several completed ones I've read before:
Kaijuu no Kodomo/Children of the Sea
Genre: Seinen/mystery/supernatural
Comments: This one has a movie too. Very pretty art.
Aoi Uroko to Suna no Machi/Mermaid Scales and the Town of Sand
Genre: Josei/mystery/slice of life
Comments: This is very short (11 chapters) and has an interesting/cute art style.
School Ningyo/School Mermaid
Genre: Shonen/horror
Comments: This is kinda a cult horror manga classic with interesting visuals. It's more so a series of one shots than a proper narrative.
Crimsons: Akai Koukaishatachi/Crimsons: The Scarlet Navigators of the Ocean
Genre: Shonen/adventure/educational (kinda)/comedy
Comments: This one is pretty silly and stupid. It's an adventure story about the life of salmon.
Umi no Cradle/The Cradle of the Sea
Genre: Shonen/Historical/gender bender
Comments: This is a navel adventure story that's pretty short (23 chapters).
Suiiki/Waters
Genre: Seinen/fantasy/historical
Comments: This is more about rivers than the ocean. It's by the same author as Mushishi, so the art is stunning.
Silent Blue
Genre: Josei/mystery
Comments: More about a lake and also very short (6 chapters)
Ningyo Ouji/Mermaid Prince
Genre: Shojou/psychological
Comments: This is 3 chapters and the first story in an anthology.
Kaikisen/Return to the Sea
Genre: Seinen/mystery
Comments: You might want to find this elsewhere since not all the chapters are here. It's Satoshi Kon.
Gyo
Genre: Seinen/Horror/Psychological
Comments: Juju Ito sea horror
Aqua Knight
Genre: Seinen/action/fantasy
Comments: Not really complete (indefinite hiatus). This is Alita Battle Angle but with orcas.
Suisei no Gargantica/Gargantia on the Verdurous Planet
Genre: Shonen/mecha/action
Comments: This has an anime. It's like if they added mechas to the movie Waterworld.
I haven't read a lot of these in a very, very long time, so my memory may be a bit hazy. Hopefully something sticks out to you!

Try this one shot !
https://www.mangago.zone/read-manga/the_specific_heat_capacity_of_love/

https://www.mangago.zone/read-manga/spirits_of_the_sea/ Second times the charm
Anyone know of any good plot oriented manga with an aquatic theme? Like sea monsters, ships, or just the ocean in general.