Waterproofness is not an inherent poly cotton property (in other words, the material doesn’t start out waterproof; it has to be made that way).
Poly cotton fabric can be made waterproof by laminating it with a waterproof membrane (GORE-TEX, for example). Alternatively, you can impregnate poly cotton with a water-repellent/waterproof coating, but doing so can severely impact the breathability of the garment.