We have a couple of vintage fiberglass chairs that have very faint water stains. Apparently, the chairs were left outside at some point in their lives. Water pooled on the seats and discolored the chairs. Sorry, I couldn't get the discoloration to show up in the photo.
In a local hardware store, I noticed a product that is a "polish and rejuvenator" for fiberglass, marble, and acrylic. It claims to eliminate water spotting and staining. I bought the product and immediately brought it home to see if it would remove the stains. The instructions say to apply the gel, wait for it to haze, and then buff with a soft cloth. This sounded easy enough, just like waxing a car. I followed the instructions, and could see some improvement as soon as I applied the product. I was hopeful that the chairs would soon look as good as new.
The water stains didn't come out as much as I hoped they would. The stains are less noticeable, but are still present. Although the cloudy white stains didn't go away, the chairs are very glossy and look better than they did before.