Some say it works but mine didn't. In anything except a mild rain my boat would be soaked. I treated my cover two years in a row without an improvement. I loved the way that cover fit but it left marks on the seats and wasn't much of a cover when it came to rain. I ended up replacing it with a cover from skiboatcovers.com and haven't had water problems since.
In the end it was a great dust cover and not much more, at least that was my experience.