This is a very timely thread as I have the exact same problem. My boat is a 1976 model and by the looks of the glass, the water spots are 40 years old as well.

I even have them on the windshield frame. If the 3M works on the glass, I will try it on the frame as well. I wonder if one of those stiff foam pads for my buffer would work on the frame and the corners of the glass.