Yes, I agree that the coating is more a marketing issue than reality. The UVC wavelengths are from about 100nm to just under 300nm. The energy per photon at 100nm is about three times higher than it is at 300nm so that's why the UVC has germicidal properties.
I'm not really sure about saltwater, but our vision also fails to record data at UV wavelengths, so I'm not sure whether the corals are fluorescing the UV back into the visible spectrum, whether the corals develop pigments to reflect the UV, or whether the bulbs emit partially in violet and that we simply are seeing the reflected violet/indigo.
Again, when we are looking at objects illuminated by black light, generally we can only see the reflected violet/indigo. We can't see the UV. The UV-A/B lights used in the way you mention have a negative effect on the plants but it's simply that the plants are able to adapt and repair the damage or to develop pigments which ameliorate the effects of those wavelengths. I would be much more worried about what those lamps are doing to the eyes. Forget about the plants.
Cheers,