If you have some harder to remove marks, or light scratches/wiper marks (usually from inadvertently hitting the wipers on a dry and dirty screen, which I do a bit as you mix up the light and wiper stalks from driving so many different cars!) It's fixable. Just apply the Metal Polish with a machine, using either a very hard foam compound pad, or a purpose specific glass polishing pad. Apply onto the windscreen, then turn the machine on at its slowest speed (so you do not spray you and the local area or shed in the polish compound) gradually turn the machine up to just past half speed and move over in a series of passes top to bottom, side to side, making sure you get a blanket coverage, taking extra time on the areas the marks may be in. Working on about a quarter of the screen at a time, I find about 4 to 5 passes is usually enough. Once you're done, turn the machine off while it is still on the glass, pull it off and then clean down with a clean microfibre buffing cloth as above.
Harder to remove water marks might need a few applications, however we have seen some severe water marks that are virtually impossible to remove without damaging the glass, which means if this does not work, you will need to look at replacing the glass, but for all of but the most evil of cases this method works just fine.