ADDENDUM:
Behe's asserts that systems are complex because he requires them to be complex for his argument to hold water. His failed mousetrap analogy ("a mousetrap requires five parts") was shredded by another scientist wearing a mousetrap with two parts removed as a tie clip. His whole premise requires that simpler systems lack utility.
DNA and the amino acid tryptophan absorb specific wavelengths of UV. This is an intrinsic feature of the molecules. DNA is mutated by this process. There are numerous compounds that absorb the same wavelengths. A few feet of water works very nicely, but there are other solutions that would enable life to exploit shallow water. A number of molecules, especially those which have conjugated double bonds, such as some polyunsaturated fatty acids, have that property. Those compounds are often effective antioxidants, as well. The ability to produce or concentrate such pigments offers an advantage independent of vision. On the other hand, once that molecule is present, coupling it to a protein might allow other benefits, not necessarily vision. For example, a related protein to the visual pigment rhodpsin, bacteriorhodopsin, fixes carbon using light energy.
The interdependence of the molecules is a function of the refinement of a system. There are suburbs in which people are totally dependent upon their cars. The car was not developed to create the suburb, nor was the preexisting farm town slated for that purpose. Because people had cars and there was cheap land, people populated the community. The small grocery store in town, where land values were higher, gave way to the supermarket on cheap land at the outskirts of town. It was at that point, that everyone had to drive. Behe ignores life that shows the intermediate benefits (e.g. UV protective pigments and bacteriorhodopsin) and in the analogy would ignore the small in-town grocery store and farming community, even though other towns resembling the earlier community-types still exist.