Sayonara Lara

Lara is a mermaid princess who once crossed into the human world for love and vanished into the sea. Two hundred years later, Lara wakes in Lake Biwa and gets another chance to search for the love that could keep her from disappearing again. (MyAnimeList)

A human girl, Mari, becomes Lara's first guide to modern life. The premise gives the romance a clear clock, while the contrast between mermaid folklore and an ordinary lakeside town supplies the hook for episode 1.