If there is no other way to store the water securely I'd probably bury it underground. I figure it's the best way to keep it out of sunlight, wildlife and nosy passers-by.

You could also suspend a water container from a tree (much like we store food overnight in bear country), but I would only risk it for a very short term cache. In my experience, any little bit of gear or supplies left out in the woods in a visible and/or easily accessible location will not last very long.