Left a clear plastic bottle of water half full in the boat at work next to the window, came back 2 weeks later, it had some algae and green stuff in the bottom of it. I was actually quite surprised, not seen that before, not that I was relying on it being fine or anything. There again it was pretty warm as it was sunny and all the windows were shut and the bottle had been used again and again for quite a while and certainly was about as far from sterile as you could get at the beginning.
In addition to all the stuff others have said I would have thought that keeping it cool would help a lot, bugs seem to multiply more the warmer they get. Find a low thick bough or a spot under a rock or something to keep it from both sun and warmth, pop a few puritabs in before leaving it so it's as sterile as can be, and I'm sure it'll be fine unless we are talking of leaving it weeks and weeks.
As for long term caches as someone has mentioned before, I think I would probably put more stock in gettng a water filter and gather my own as needed than hope that the water was still good and not allowed any of the surrounding humidity in after an extended period in a tropical setting. If you were prepared to dig it in to where you might find a relatively consistently cool layer in the soil then I guess it could work but it seems a lot of effort when you could just go find a stream?