I've used 3/4 water 1/4 juice once and it was wayyy too watery for me. It was not good! So I can only imagine that using only water would be...unappetizing.
I just bought an all natural orange/carrot/mango juice blend (no high fructose corn syrup, low sugar) and it tastes awesome by itself. I use it sparingly in my smoothies and sometimes have to add a bit of water, but not enough to really make it noticeable. It is def. trial and error to find the balance between liquid and ingredients.