Fishing
Because of the addition of reflected light (UV rays being relfected from the surface of the water), viewing conditions are considerably brighter than they are on land. To prevent glare, discourage squinting, prevent headaches, and reduce eye fatigue, quality polarized lenses are a must for boating sunglasses.
In addition, look for lenses that are impact-resistant and shatterproof as well as more flat than curved. These features will protect your eyes in the case of a collision and will also reduce visual distortion.