You will have to forgive having to wait for this post, I had to do a certain amount of research to make sure everything was fairly accurate.
Once again the most important factor is transmission level (T10). Water with transmission of a set level is treated exactly the same regardless of its composition. This means that both salt and non-salt water will behave exactly the same if both have a T10 of 90%. It is only when sea water goes over around 20°C in temperature that special care needs to be taken.
A salt water compatible UV unit will need to be made from 316 stainless steel, this is because of sea water’s naturally corrosive nature. The unit will need extra protection when exposed to seawater from 20-40°C. The corrosiveness comes from a layer of biofilm that forms on the surface of metals regardless of temperature. Sea water (being highly oxygenated) not only promotes corrosion between the biofilm and metal but also acts as a catalyst for chemical reactions. The one to watch out for here is the release of the Chlorine atoms from the salt, Hydrochloric acid and free chloride molecules are among the results, both of which will further aid the corrosion of local metal.
Once the temperature is over 40°C, the biofilm rapidly breaks down and normal corrosion limits hold true again. Once here, precautions for high temperature use are applied, the same as on standard UV units. If you would like to know any more about this I looked into it quite a lot and I would be happy to share.