Watashi o Tabetai, Hitodenashi

Synopsis

From Yen Press: Hinako, a high school student with a dark past, lives alone in a town by the sea. Haunted by a constant sensation of drowning, she can barely go through the motions of her tedious life until the day a monster tries to eat her! After passing out in the struggle, she awakens to find she’s been rescued by a beautiful mermaid. However, salvation comes at a cost the mermaid’s protection will last only until Hinako has matured into the perfect meal! Hinako’s first instinct isn’t to run or scream, though, and an unlikely relationship between monster and prey unfolds. “I’ve come to devour you."

Synopsis & details from MangaUpdates