UV blockage is a great feature for the health of your eyes, but it's worthless on a bonefishing trip. The truth is you can find a very good pair of sunglasses under fifteen dollars. As the quality of the lens and frame increases, so does the price. If you just fish a trip or two a year, it probably doesn't make sense to invest in a high quality pair of sunglasses. As with fly fishing tackle, buy the best you can comfortably afford.
Some manufactures have many tints available.
On the flats, an amber or brownish tint works best. Costa Del Mar. In the ocean, the blue and green tints seem to help with visual acuity. Whichever you decide to purchase, be sure they are polarized. If you are not sure, place one pair on your head and hold another pair a foot or so in front of you. As you turn the second pair, you should notice a point where you can't see through both pairs.