Wata no Soko, Okitsu Mikami no Kudanzaka Kaiki Sousho

In a town with an old legend about a sea creature that brings disaster, weird stuff starts happening. A college kid, Kudanzaka Yamahiko, who's in a club that looks into the strange and unusual, gets roped into it by a girl he met, Amano Arie. She says something washed up on the shore - not quite human, not quite fish. They go to check it out, but the guy they were supposed to meet is gone. Turns out, he's dead, and his head is missing.

[Translated from DLsite]