I've got something amazing (make sure you don't use vinegar on on marble, though, as it will etch and damage it -- vinegar is awesome for water spots in showers, though, and shower fixtures!!).
Ben A Qui! This all natural polish should get rid of the water spots on granite in no time. It is also amazing at getting water spots off shower glass. Here is a shower we did that had terrible water spots. We got rid of them with the Ben A Qui! You should similarly awesome results on your granite!