Well, clearly the water is a worse, so to say, conductor of light than air, since it possesses several properties that obstruct the light’s unhindered dissemination in it. Geographic location and the clarity of water, as well as its chemical composition are probably the main factors influencing the way light spreads in water. Other reasons include the time of day and season. Anyways, when light passes through water, it gets scattered and absorbed by water particles. The deeper you go, the more wavelengths the water absorbs, which means that you start seeing less and less with every dozen meters deeper. The longest waves (red and orange colors) get completely absorbed first, so if you want to see these colors underwater, you need to carry a special torch with you. As depth increases, light waves of other (shorter) lengths get absorbed as well. After the red color green and yellow ones disappear as well. Blue and violet colors correspond with the shortest wavelengths of the visible specter, so they disappear the last: on the depth below 500 feet your eyes will be able to perceive only blue light. If you go deeper, your eyes will not be able to detect even these wavelengths, which means that it just gets darker and darker, unless you find yourself surrounded by complete blackness (with creepy creatures inhabiting it!).
By the way, these creepy creatures, aka deep sea fishes, have eyes much more sensitive to light than humans’. These fishes can actually see in enormous depths (like, 4000 feet), which means they will be able to detect you, while you will be completely unaware of their presence. Ugh!