Well pretty much everything going through the UV will be killed if it is strong enough and if it is going through it slowly enough. Surly floating phytoplanktin and zooplankton will be killed, water become sterile.
I have looked at the water from my aquarium under a microscope and found a lot of little buggers here and there. The water become alive after sometime, from snails, crabs, shrimps larvae. I even had coral spawning and reproducing in my aquarium. So yes the water is pretty much alive and the UV kill most of that.
I never used a UV, never had a problem with any parasites because I do a strict quarantine and treat for what ever is in there as it goes. I think it is a better way of dealing with parasite than leaving them on fish in small quantity and trying to control them after with a UV. The fish will always have a small amount on them and be susceptible to outbreak.
Multi million dollars public aquariums often use natural sea water, so they don't really care for the cost of water change
So they probably need the UV to kill what ever parasites could get in with the natural sea water. Since they have a free supply of sea water, they do more water change to control the nitrates etc...I wish I could be near the ocean and get free sea water...then I would definitly use a UV to run all the water through it very slowly to make sure it is clear of nasty parasites and protozoares.
I had a UV in fresh water for my discus because they has a problem with flukes..prazi resistant flukes, but the UV never did anything for the flukes, so I sold it when I got my saltwater tank. I never needed it.
In Coral magazine in an article called "Anorexic Reef" they explained that SPS grow much faster and better in a year old mature aquarium or more because of the zooplankton living in the water and feeding the coral. I have no doubt about this from what I saw in the microscope, so no more UV for me.