Well, those dried water stains - made of calcium lime or other mineral deposits - can be wiped off easily and completely with a solution of 10% VW original windscreen wash fluid and 90% water on a clean cloth. Then use a clean dry cloth to make the glass shine.
This also removes fingertip and other greasy marks from glasses, inside and out.
Part number for the fluid is G 052 131 A1 and it cost me 50p. Then again the UK price could be different.
But it's top stuff!
Edit: spelling error

