It's the natural sugar content in onions that becomes caramelized, not the onions themselves. It can take a hour or more over a very low flame to get them dark and nutty brown--Julia Child's goal when making her French Onion Soup the traditional way. To speed things along, I always sprinkle a bit of sugar over the onions after they go into the pan. Not much... a 3-4 tablespoons is enough for 6 cups of onions when making soup, a light sprinkle (a tablespoon or so) when doing only a medium onion for a pizza topping. You'll be surprised at the difference a little sugar makes. It speeds up the caramelization process and helps to give you a nice nutty brown color.
The water comes in to prevent burning the onions during the long cooking time. At the beginning, watch over your onions and stir occasionally to prevent burning. But when you've reach the point where the onions are drying out and will do nothing but burn if you simmer them any longer, splash a half cup of water (or more if you have a large amount of onions) into the pan, and using a flat wooden spoon, deglaze the pan to get the brown bits off the bottom. Cover the pan and continue to saute the onions on a low flame. If you use the lid, you can pretty much let them saute while checking on them only periodically until they are caramelized. IF you prefer to watch your onions sauteing without a lid, you might have to splash a bit more water into the pan from time to time.
If you use these two hacks, instead of over an hour, you'll have sweet caramelized onions within a half hour.