Waterproof sounds good in theory, but no stiched fabrics really are, and the closer they are to waterproof, the greater the odds they will trap moisture, and mold and rot the seats below, as well as rust the frame.
What you want is breathable, water resistant fabric ..Bartact is top of the line , but CalTrend and Over are out there as well , and we have seen some good threads on these.
I have the Bartact, and have been completely satisfied. Had to swallow when I saw the price, but no regret later.