Hatsukoi no Kaeru Shima

"I've been in love with you for so long that I can't even remember when it started..."
Tatsunoshin, an aspiring cook, and Nanami, a diver, are close childhood friends who, before they knew what happened, were even masturbating together. For the sake of changing how he's always doted on by Nanami, Tatsunoshin decides to leave the island. However, Nanami replies, "Leaving won't change a thing, You'll always be dearest to me," and refuses to give up.
(Source: Frontier Works, translated)
Note: Includes 1 extra chapter.