Float Boat Harris 22 How Do I Make A Boat/ship Light Enough To Float?

How do i make a boat/ship light enough to float? - float boat harris 22

I remember something about how some of camphor at the base of a cut sheet of plastic in a small boat base. when placed in water, they move. Does anyone have any information about this topic?
What can I use for ships and floating vessels / fish for a child?
all ideas and suggestions are welcome
Thanks