The boy was spotted floating unconscious in Henry Hagg Lake late Monday afternoon, police said. Attempts to revive him were unsuccessful.
Washington County sheriff's deputies trying to locate the parents found beach and picnic gear and a leashed dog. Two adults and a child were missing.
The family's car was in the lot.
Authorities said the boy was not wearing a life jacket when he was found. The lake has a rack at the entrance with child-size life jackets, which are given out free.
Police have not released the identity of the boy or his missing family, described as two adults and another child. Other relatives have been located and now have the dog.
Visibility in the lake is bad, which makes underwater searches difficult.