In some ways a fish's eye is comparable to a human's. Their color vision, during bright conditions is very similiar to human's. Also like a human's, light enters the fish's eye and is picked up by the lens at the center of the eyeball, the light is then focused to the back of the eye onto the retina. Although I've never heard of fish being given a vision test I think it might be a safe assumption that some of them would need glasses if they were human due to normal eye deformations. Wasn't there a movie about that? Don Knotts comes to mind.
In the human eye the iris opens and closes depending on the amount of light entering the eye. The iris in a fish is fixed. It is also opaque except for the small hole in the center. Light can only enter that clear area. Receptor cells in the retina (back of the eye) make the adjustments for differing levels of brightness by moving in and out, and they do it fairly slowly. This might explain why fishing can be so bad on partly cloudy days where the light level changes suddenly and frequently as clouds cast moving shadows. Under these conditions the light level changes so frequently, and the fish can't rapidly adapt, so they just hide.
There are two types of receptors in the fish's retina, rods and cones. Each type sends a different kind of signal to the fish's brain describing the image flashed on the retina. The cone cells receive color signals so fish see in color in the daytime. At night the rod receptors are used. The rod receptors do not detect color, but are 30 times more sensitive to light than the cone receptors. So fish see colors as well as humans in the daytime, but are color blind at night and see far better in the dark than humans.
Predator fish generally have far better vision in low light levels than their prey fish. This gives the predator fish a larger advantage at night over their prey than in the day time and could partly explain why fishing very early and very late tends to be more productive, because that's when they are actively hunting. Night fishing tends to also be productive.
What we call crystal clear water is still a poor light conductor compared to air. Visibility underwater is limited substantially under the best conditions, but when there is silt, mud, vegetation particles or heavy color to the water light really scatters and is similar to our problems seeing in fog. It doesn't matter how good your vision is if light is scattered heavily. So fish don't generally see anything except up very close. This doesn't appear to apply to what they can see above the water, if they are in shallow water. We've all had them spook at a distance fromt that.
So under normal conditions fish can't see more than a few feet. Unless a fish is alerted by sound, vibration, or smell to the approach of a lure it may have only a very brief moment to strike or otherwise react when a lure or fly suddenly appears in it's range of vision. In dirty or cloudy water that distance may be only a few inches. Added to that a fish's eye can not flex to change focus. Fish often use their other senses to locate prey and their vision only comes into play at the last moment.
Because their eyes are located on each side of their heads they have a wide range of vision, with only a small blind spot directly behind them, but their eyes have overlapping fields of view to the front. Directly in front they have binocular vision, but lose that to the sides. In front they have depth perception, yet clarity of vision diminishes towards their front because to look forward they are looking through the edges of their eyes, a binocular peripheral vision so to speak. That depth perception diminishes to their sides, but their clarity of vision increases to the sides because they are now looking directly at the object. So to see an object clearly they have to look from one eye only, and to judge distance to that object they have to face it head on. I've seen many fish look at a bait side on before they turn and strike. This may be why.