Untreated water can contain bacteria and other microorganisms that can mess up your day or take you out of the game. Take the time to treat your water and save yourself from backdoor trots.
Boiling Will kill everything is an easy and cheap way that requires no special equipment. There is however, a challenge and that is you have a source of heat to boil the water.
Chlorine dioxide kills everything and is cheap, easy to use and light to carry. One of the most important qualities of chlorine dioxide is its high water solubility, especially in cold water. The 4 hours and 30 minutes that you have to wait for it to kill Giardia and to neutralize cryptosporidium can be used for other activities. Oh, did I mention that there is a taste?
Filter Kills bacteria and protozoa fast with no funny taste but water filters can be bulky, pricey and will require field maintenance.
Ultraviolet light I have never used one of these personally but Ultraviolet light devices kills everything fast, light to carry and is not supposed to leave a taste. Ultraviolet lights are pricey and requires batteries.