Hatsukoi no Kaeru Shima

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 doting on Nanami, Tatsunoshin decides to leave the island. But Nanami won't give him up, saying "Leaving the island won't change that I'm the most important person to you."

(Source: Frontier Works, translated)