I removed the water spots from my rear window on my roadster. I tried to use a hard water spot remover, I've tried this before on the other windows, it still doesn't work.
I used Griot's Garage Complete polishing compound. Don't use a lot of pressure when polishing the rear window. The glass could fall in or break. Pack pillows blankets, etc.. under the glass to help support it.
Here are some pics from my facebook post: