UV and protection filters can and do degrade image quality. The usual problem is flare and ghosting rather than any loss of sharpness, though that can happen with poor quality filters and is more noticeable on longer lenses that magnify imperfections.
The sensible thing is to use a lens hood, that can only enhance image quality in difficult situations, and offers excellent physical protection. Only fit a filter for protection when you need it, ie when there's a real danger of something nasty coming into contact with the lens, eg sea spray, dust or sand, flying mud etc. If a filter gets wet, don't let the water dry - wipe it carefully (microfibre cloth). Dried-on water marks can be pretty much permanent, unless the filter has an extra easy-clean stain-resistant coating as many of the best ones do.