Let's☆Lagoon

Yamada is a castaway on an island without any ships passing by and he is unable to recall why he is there. After finding his sports bag, he assumes he was on a school trip. With only 7 days worth of food, he eventually decides to make a boat to leave the island. He thought he was alone on the island until finding a girl named Imaise Chika eating his food.

Volume 3 contains chapter 30.5.
Volume 6 contains an epilogue.