First, I wouldn't be especially worried about the odd fingerprint, dust mark, or water on the lens as it would take quite a bit to make an impact. To give you a sense, helpfully posted in another thread is this lens which, despite massive damage to front element, still works and does so reasonably well.

Second, there are ways to reduce your incidental contact with the lens such as using the lens hood and ensuring you cap it when its not in use. If you do get something on it, despite that, then cleaning tools such as the lens pen and microfibre cloths will help you remove it. That's if you even notice!

However, if you do get something like a UV filter, which will help with this problem, bear in mind that you still have to clean that with reasonable care or you introduce potential image artifacts beyond what the filter will already do. By the way, the downside really applies to both options in a way, primarily artifacts from light sources appearing in unexpected places. It is, after all, another piece of glass added to your lens.