Never use just water to wash your car - water doesn't have the needed lubricity to remove dust, dirt and mud without marring. Longevity varies depending on the product, some last longer than others, which is why most people choose a more durable product (the Collinite waxes) rather than less-durable products. That said, something like Collinite will definitely not "wash away" with a normal wash (using properly diluted car shampoo).