Anything buoyant -a polystyrene box, or even an empty, tightly sealed plastic bottle - can help someone stay afloat.
If your trousers are made of a natural fibre - cotton or wool - take them off and knot the bottoms of the legs. Holding the waistband, swing the trousers over your head and down onto the water. Then tuck the air-filled legs under your arm-pits to serve as a buoy.
Caution Do not remove your clothes in icy water.
