Although this topic has little application underwater, virtually all of us also use our cameras topside, and after responding to a question on this topic over on DigitalDiver.net, it occured to me that it would be interesting to get the input of Wetpixelers on the subject. Here's most of my post from DD:
I recently attended a two-day "nature photography" seminar in Orlando put on by John Shaw, a well-known nature photographer (www.johnshawphoto.com), and experienced a total paradigm shift on the subject of UV filters.
I have always religiously purchased a UV filter for every lens I bought, because someone once told me (back in my film SLR days) it was crucial to protect the lens from getting scratched. In fact, I've often kept UV filters on lenses inside camera housings underwater (how bright is that?!) because somewhere in the back of my mind, I was worried about scratching something...I guess when moving the camera in and out of the housing. Despite my doubts that the "UV light correction" aspect of such filters was really of any value (I just figured "it can't hurt"), I bought into all the FEAR OF DAMAGE propaganda...hook, line and sinker. 
In his seminar, John Shaw said (I'll attempt to paraphrase accurately) that he thinks buying/using UV filters is a huge waste of money. 
He said he has gotten rid of all his fancy filters EXCEPT polarizers because anything you could do with other filters you can now do in Photoshop after the fact (assuming you shoot RAW). Because a polarizing filter is SO useful in nature photography, and its effects CANNOT be duplicated in Photoshop (or any other editing software), he considers that THE essential filter, on the condition that you buy the best one you can find (i.e., Nikon or Canon BRAND), not some cheap off-brand. He did say that occasionally he sees a use for a graduated ND filter, but that's the only other kind of filter he owns now, and he seldom uses it. In general, he either uses a polarizer or no filter at all.
As for needing a UV filter to "protect" your lens, he said:
A. he is very careful with his lenses, and if something happens that damages the front element (e.g., dropping it), it will likely have damaged a lot more than a UV filter would have protected, and;
B. the cost of replacing the front piece of glass in most lenses is not much more than the cost of a good UV filter, so for all the money you would have spent buying UV's for all your lenses, you can easily afford to replace a scratched front glass now and then, and finally;
C. why put an extra piece of glass between you and the subject if you don't really need to?
Well, let me tell you, I was stunned! It was as if someone suddenly told me all that talk about the earth being round was a myth, and it had now been proven to be flat! But the more I've thought about it, the more sensible his position seems. I've never scratched the UV filter on the front of a lens; I've just spent a lot of money on them (I coulda bought several more lenses with that money!)!
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