My opthamologist told me that if I live long enough (most people do) I'll be thankful I protected the quality of my vision by wearing sunglasses whenever I'm in sunshine. If I squint at all when I'm outside I put on sunglasses. My Solar Shield sunglasses (about $20 US in a store like KMart) are large enough to be worn over my regular glasses and they wrap-around so they provide some side protection too. I tie a long loop of string to the bows and keep them attached by putting the top snap of the my life jacket through the loop.
When I get a new pair of regular glasses the old pair becomes the pair I wear on the water under sunglasses. So I'm not risking my most up-to-date glasses when I windsurf; I'm not paying the extra expense of prescription sunglasses; and the combination of Solar Shield sunglasses over my older glasses gives me protection & clear-enough vision.