From JShoujo Scans:
After three years Mimi comes back to the island where she had spent her childhood. She is greeted by her childhood friends Yousuke, Sumire and Hikari. Heartbroken in Tokyo, she has now came back to the beautiful island surrounded by the deep blue sea.
But among this four friends, friendship soon turns to love…!? A blue world spreads before your eyes, an unforgettable story begins.
Blau - Wie Himmel, Meer und Liebe
Author & Artist: CHIBA Kozue
Genre: Drama | Romance | Shoujo | Slice of Life
Status: 7 Volumes (Ongoing)
Vol. 4 - Mirror
Vol. 5 - Mirror
Vol. 6 - Mirror
Vol. 7 - Mirror