Cocoon

On the surface "Cocoon" (1985) is about elderly folks at a retirement home in Florida unknowingly finding the "fountain of youth" via a pool on an adjacent property. A peculiar group of people rent the property to store boulder-like objects they take from the bottom of the ocean, acquiring healing powers that restore their old folk's youthful vigor. The film stars Steve Guttenberg as the likable protagonist, a boat owner/operator who helps the people retrieve these objects without knowing what's really going on.