KIMI NO ONDO NI SOMARU RURI: HIKARU YORU NO SHIZUKU

Niizato Gara, who runs a glass workshop inherited from his grandfather, falls into the sea while picking up discarded bottles. The last thing he sees before losing consciousness is a glittering, glass-like object... a mermaid!? Gara awakens in the arms of Ruri, a mysterious mermaid who came from a distant sea. They are immediately attracted to each other, but Ruri becomes excited when Gara touches his shiny scales...

(Source: Beltoon, translated)