why did the author leave the kids naked on the beach what does this say about their civilation

The boys disrobe and swim naked because they feel a sense of freedom, something they've never had before. Parental/ adult supervision had always been a part of their lives..... swimming naked is a celebration of their freedom from adults. You might see this as the beginning of the deterioration of civilized behavior, or you might just see it as boys being boys, as all civilized behavior will be quickly lost.