Kaijuu Iro no Shima

Middle school student Kon Chigawa has never left the small island she was born on, nor does she expect to. Shunned by her classmates, Kon's always alone, and she hasn't ever been in love, either. That is, until she encounters the city girl Furuka on a cliff-and ends up resuscitating her after they fall into the ocean! Upon waking, the two mysteriously find themselves in front of a cave said to house monsters, a place Kon often spends time by. And with that, strange occurrences begin to happen on the island...

(Source: Yen Press)