A lost sailor meets a lonely mermaid.
Or:
A sailor exploring the open seas gets struck by a storm and washes ashore on a
a remote, mysterious island. She is found by a mermaid who appears gentle but lonely, and the two hit it off while the sailor works on fixing her boat to get back on her adventures. As the days pass, the sailor gets more and more used to the slow-paced, quiet but peaceful life on the beautiful island and the mermaid’s company - but she knows that at one point, she has to leave it all behind.