What happens if you only drink fruit juice over a long period of time? and is it best to substitute that for water instead?

Too much sugar?: Most if not all fruit juices are high in sugar content, often as high as soft drinks or even higher. Substituting juices for all water intake probably means you're comsuming too much sugar. This is a particular problem if you are overweight or prone to diabetes (e.g. family history). High sugar intake also promotes tooth decay. I suggest you cut it down.