Let's☆Lagoon  

レッツ☆ラグーン, 렛트 ☆ 라군, Манга Лагуна

Yamada is stranded on an island with no passing ships and cannot remember why he's there. He finds his sports bag and assumes it was for a school trip, even though he only has enough food to last 7 days. Eventually, he decides to build a boat to leave the island. However, he discovers another person named Imaise Chika who is also stranded on the island. They decide to work together to carve the boat. Since there's no room for both of them in the boat, Yamada sets off to find help but returns when he notices that Imaise has put her share of rations in his bag and they are mostly untouched. After returning, he realizes she made an oar out of a fishing lance, which means they can survive on the island longer.