Why and for which cameras ?
The UV- Filter (ultra violet light filter) will remove the invisible UV-light and the haze occasionally resulting from this light. Results are normally crisper and clearer. The filter is also a perfect protection of the lens from fingerprints, scratches and general dirt.
The UV-filters are not necessary for underwater use, as the UV-light is the first spectrum to be filtered out of the light after only a few feet.