Baking soda and vinegar works wonders for getting water stains off of mirrors and other glass.
To get water stains off your mirrors take equal parts of baking soda and equal parts of vinegar to make a paste and then apply the paste to the glass mirror and let it sit for around 10 to 15 minutes and then use a brush, sponge or towel to lightly scrub the glass mirror to remove the water stains.
Then spray some glass cleaner or water onto the mirror and wipe it down again.
For really hard water stains on the mirror or other glass you may have to repeat the procedure.