This Boy Caught A Merman

Shima jumps into the ocean to retrieve a picture of his late grandfather, but nearly drowns in the process. He's saved by Isaki, a young man who turns out to be a merman from the waist up. Despite their vastly different species, they form a strong bond, which would be challenging to maintain due to their genetic differences. This contrasts with Shima's own feelings of being an outcast.