Polypropylene rope floats, and it is the only kind of synthetic rope that does! The cheap, wirey, yellow braided rope is polypropylene but don't buy it. Get good marine or rescue grade polypropylene ropes that are strong and hold knots. The most common use is for painters (end ropes) and throwbags. The best rope diameter is 8-12mm so you can hold on to it easily with cold hands.