Kaijuu Iro no Shima by Hattori Mitsuru — Ongoing
Without a single friend, middle schooler Chigawa Kon lives on a small island and laments that she hasn't "fallen in love" yet, unlike her classmates. She often plays by "the Hole", a cave near the sea where a man-eating monster lives, according to folk tales.
One day, as she meets with another girl who accidentally drops her smartphone in the sea, they fall from a cliff together but miraculously survive. It turns out Furuka is also all alone.