Let's Lagoon

Let's Lagoon manga summary:

Yamada is stranded on an island without any ships passing by who isn't sure why he's there because he can't really recall, so after finding his sports bag, he assumes he was on a school trip. With only 7 days of food, he eventually decides to make a boat to leave the island. He thought he was alone on the island until a girl named Imaise Chika was found eating his food, where they then decide to continue making a boat. Since the boat only had room for one, Yamada decided to set off for help, but returned upon noticing that not only did Imaise put her rations in his bag, they were mostly untouched as well. After returning, he noticed she made the oar out of a fishing lance and he decides they can live on the island longer.