Seal the joint on the hula hoop with plumber's putty. Inspect the hoop for any holes around its length and fill these with the putty. Wrap plumbing tape around the joint and around any other areas with filled holes so water can't enter the hula hoop tube.
Lay the hoop on top a sheet of 6-mil black polyethylene film, which is the plastic commonly used for gardening and landscape projects. Cut the plastic film so it is 1 inch larger than the hoop on all sides.
Run a bead of waterproof epoxy glue on top the hoop. Lay the black plastic on top the hoop and stretch it taut. Clamp in the plastic to the hoop as the epoxy dries, which can take up to 24 hours.
Remove the clamps. Cut the excess plastic off where it overhangs the hoop.
Punch a hole through the center of the plastic with a nail. Punch eight additional holes in a circle halfway between the center of the plastic and the hoop frame. These holes provide a vent so air doesn't become trapped beneath the lily pad.
Float the completed lily pad on your pool. Make enough to cover at least half the water's surface to experience optimum solar heat gain.