Attach a line to lower and raise the light in the water. The amount of line will depend on how far down you want to set the light. It is better to have too much than too little, so use at least 20 feet. Wrap the excess line securely to keep it from knotting. With a plastic bag, tie the line around the outside of the bag and flashlight. Tie it tightly so that it won't slip off. With a plastic container, you might need to cut a hole in the handle to connect the line. With a waterproof valuables container, just tie the line around the loop or clip that comes with it.