On Land:
For protection, a UV Haze filter should be on every lens you own. Buy one that is along the same quality lines as the lens itself. In other works, if you are using a Canon L lens, you should be using something like a B+W lens for protection. If your lens isn't professional calibre, a lesser protection lens will be okay.
The only other filter I use these days is a polarizer. I use a circular polarizer and I have one each for every lens diameter I own.
When you purchase a lens, you should be purchasing the protection filter and the polarizer (if you need one it that diameter) at the same time.
Pretty much everything I once used lenses for I now can do in Photoshop.
For underwater:
I normally remove the protection filter. The camera housing will serve as protection. Some wide angle lenses require magnification, called a diopter. This is actually another lens, not truly a filter, although most people would consider it a filter. The diopter is required specificall for wide angle zooms. Check with the housing manufacturer about which they recommend with a particular housing.
Jeff
