NOTE: The coil can get up to 745C, as per the specs. (Failure after 800C, 1073K, 1472F.)
Heat-color chart. (I see cherry red when it is dry.)
http://911research.wtc7.net/cache/wtc/analysis/fires/metcolor.htm
With a wet wick, it should have a hard time getting over 200C. The water/evaporating fluids, carry away heat in the negative pressure chamber. (You create negative pressure as you suck on the vaporizer.)
PG and VG are only in the solution to create visual effect of smoke. (Fog machine fluid.)
Being that the melting point of poly-fill is 260C, and that is less than Glycerol (VG) 290C, you would gain from using water and glycerol. You have less chance of melting your cartridge wick. The water would steam-off first, and take some glycerol with it. This would reduce the viscosity too, allowing it to flow like Propylene Glycol (PG) 188C.
PG and water might be better for your health, if PG causes irritation, and VG causes undesired digestive issues. The water will be the carrier for the PG, but the viscosity may be too thin, and require a thicker wick to reduce flow.
In any event, mineral water is bad, and will cause undesired deposits that may short-out the coil. Distilled water should be better, but most distilled water is mineral enriched, to put back what was taken out!?!?!
You will have less "Visual Vapor", but you will actually have more vapor. You just don't see the water vapor, as it condensates faster in your mouth, and evaporates faster in the air. (It is there, just not large enough to diffuse the light, as a thick PG or VG cloud is able to do.)
That being said... it will also condensate more in your vaporizer air-chamber. This could lead to undesired dripping or liquid ejection when drawing on the device. If you have this issue, try less water.
Most bottled water is just tap-water from another location. Some say distilled and enriched, some say reverse osmosis and enriched, some say distilled and reverse osmosis. It is the "mineral enriched" water that you want to avoid. (As well as any mineral water, vitamin water, health water, etc... which have an ingredient list on the bottle.)
I imagine a nice neutral mix would be...
1/5 water (To restore the undesired high viscosity which VG creates)
2/5 VG (To reduce PG irritation)
2/5 PG (Original mixtures)
The water also helping to aid the removal of the other fluids, before the temperature is high enough to melt the poly-fil wick in the cartridge. (Using the 1/5 portion for any added flavoring too. Resulting in less water used.)