I was thinking about a mile from a natural body of fresh water, it'll start lightly pulsating. The closer you get, the brighter the light gets and the further you get, the darker it gets. It can be neutralised by being placed in the water, in which the light will fade until its close to nothing. If we're gonna be trudging through forests and the like, there won't be much use for it but if we're in areas with a lack of water (desert, mountaintop, etc) it could be useful.