Eiji, a member of his school's swimming club, attends a high school on the mainland separated from his parents and family. For the summer the club decides to visit Sumire Island, Eiji's home town, for a mixture of training camp and vacation. Eiji is glad to see his mother again and meets lots of old friends and acquaintances.
However, in recent years Sumire Island has become a hot spot for summer tourists and is overcrowded, so the group has trouble finding a place to stay. Luckily, they all can stay at Eiji's place and even decide to start working as life guards, helping to protect the crowded beaches.
Eiji's exciting summer can now begin.
If you suspect a nukige, you're right even though it's not 100% clear from synopsis. Can't even call it a moege since heroines aren't moe.