I experimented with this stuff before WeatherTech was a thing. I saturated a set of new carpeted floor mats with it, and let them bake in the sun for a day.
Water never soaked into those mats again. Whenever I jumped into the car with wet or snow-covered shoes, the water beaded up and sat there until it eventually dried. If I pulled the mat out, the water would roll off. I'd say it definitely works, but don't be stingy with it.