After UV protection, look for polarization to reduce the glare from the water. That's the canyon cutting through the reef. If you couldn't see a thing, it was probably because you weren't wearing polarized sunglasses. Another nice feature is optically correct lenses, which are distortion-free. Distortion, like glare, won't damage your eyes, the experts say, but both can give you a headache at the end of the day.
Plastic is less likely to break, more likely to scratch.
Even salt crystals can scratch some plastic lenses. Likewise, flexible frames are less likely to break but can't be adjusted to fit well. Try Wal-Mart if you don't believe me. Good question, particularly when everything on a dive boat seems to find its way under someone else's weight belt. Credibility for the maker's UV claims.