1. Water streaks
Well it depends on how long its been exposed to the elements, if its baked in you have no choice but to use a machine and a glass cleaning compound to remove the watermarks.

2. Swirl marks
You have two choices either use a polish and machine to level the swirls or fill the swirls with glaze and wax.

3. Light scratches
Try using 3M scratch remover, remember this is only for spot threatment.