Let's☆Lagoon

Let's☆Lagoon (レッツ☆ラグーン) is a manga written and illustrated by Takeshi Okazaki. It has been serialized in Monthly Young Magazine's magazine from October 22, 2007 to October 20, 2018.

Plot
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. They then decide to continue carving the boat together. Since the boat only has room for one, Yamada decides to set off for help, but returns upon noticing that not only has Imaise put her share of rations in his bag, they are mostly untouched as well. After returning, he notices she made the oar out of a fishing lance and he decides they can live on the island longer.

Other Sources

 * (MAL)