The most important thing is that they are polarized. Whether or not they filter a certain spectrum of the sun's rays is not as important as their ability to remove the glare from the surface of the water. With an increase in price of those glasses one can expect a higher quality lens with less distortion, more lens color choices and a better polarization lamination. The ability to protect ones eyes from harmful UV rays also is expected of a quality lens. Generally most guides here in the Keys choose an amber or a vermilion tint for the flats.
Some guides have multiple pairs for different lighting conditions.
A bright yellow tint for early morning or low light days and a darker tint for brighter, sunnier days. For those deprived of perfect vision, there are a few manufactures that offer prescription polarized glasses. Costa Del Mar frames and vermilion prescriptive lenses from Costa ground by Island Optics in North Carolina. One quick tip to tell if a pair of glasses is polarized. If you take a pair that is polarized or suspected to be and place it in front of another pair, slowly rotate one pair ninety degrees while looking through both pairs.