Sailors who have lost their boats at sea and end up having to stay alive in the ocean, or pilots who have had to abandon their aircrafts over the ocean may sometimes use a shark screen bag. 

The Shark Screen is like a plastic sleeping bag that floats in the water.  Most shark attacks occur on easier to bite areas of the human body, like arms and legs.  A person inside the Shark Screen looks like one big piece and does not give the shark an easy target to bite.

The shark screen bag hides the person's shape and keeps the smell of the humans from reaching nearby sharks.   Also, blood cannot escape into the water, which attracts sharks to humans.