If you're looking for manga similar to Muromi-san, you might like these titles.
Nagisa has just recently transformed from being a human to a mermaid and is easing into the transition of living in the ocean. But when she crashes into a research submarine, she loses her memory and is brought on board the Sogei. It turns out that her love interest from when she was still human, Osanai, is on board and he tries to keep his past with Nagisa a secret from everyone else. Her mermaid friend, Tsunami, goes on a rescue mission to find Nagisa and warn her that since she just transformed into a mermaid, if she doesn’t speak for an hour, she turns back into a human and her life expectancy will dramatically drop!
The underwater world and the real world are linked to one another. Sango, a hospitalized girl whose name means coral, makes up cute, mysterious stories about the underwater world.
The tale of a boy turned into a merman by his father and released into the sea, raised by a turtle, who befriends the daughter of Captain Nemo (who kills the turtle) and a piano playing fish who befriends a talking, underwater car in an attempt to save Nemo’s daughter from her own father because she swallowed a red pearl by becoming a member of his crew…
Azuri, a mermaid princess, is betrothed to Prince Unagi. Their marriage is arranged to bring peace to their two kingdoms, but when Azuri falls in love with Thale, a royal guard, she doesn't know if she should save the world or follow her heart.