Standard fabrics are : 100% Polyester, 100% Cotton or Mix 65% Polyester & 35% Cotton. The fabrics have a water resistant coating. They are not water proof, but will repel water in moderate quantities. However, if the cushions are left out-doors in heavy rain for more than one day the fabrics will start to absorb water.
To make the cushions as close to 100% waterproof as possible the cheapest way is to use waterproof Inter-liner and a water resistant polyester fabric as shown on the Deck lounger below. Polyester fabrics are better water repellants than cotton.