A car cover can never be 100% "waterproof". Even if a cover has the ability to keep water from coming through the cover, condensation can still get through the cover through a process called osmosis. There are also car covers that are "breathable". Breathable covers allow moisture to pass through the cover but also air, and the transference of air through the cover allows the moisture that is on your vehicle to dry faster than a non-breathable cover. Non-breathable covers will trap moisture on your paint finish, which can damage your paint, and in worst case scenarios: RUST.
Seven waterproof and water-resistant fabrics: