Aoi Umi no Sashimi

Overview

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!

Source: AniList